Looks like plain old SIMM, like what a Macintosh LC would use.

If you can't find the specs of RAM on line, you need either a magnifying glass or to work on your google kung fu.
Those modules are "PS2" or "72 pin" and should have 8 MB each. You'll need two identical ones with a pentium.
All intel pentium chipset mobos have those. (Some later ones also have other slots.)

One good photo would have made it much easier, but I assume it's M5M417805 and this can be found:

I'd say get two pairs of 16 MB modules, those should be priced in the single digits of $ or €.
No point in installing more than 64 MB here.

FPM will work but the Intel FX chipset does take EDO:
https://en.wikipedia.org/wiki/List_of_Intel_c … entium_chipsets

The hard disk, well, sometimes they're tough.
As long as you get the cable off the motherboard, after removing the RAM for better access:
Remove the drive from the case first.
Could help if the cable is expendable. Don't have too much concern for it, they're cheap.
Also, make sure it's not glued down somewhere.
Never had to try, but maybe some spray lubricant could help.
If you don't want to risk damaging anything, leave the cable attached, use something like this to attach the USB bridge to the cable:

Motherboard seems likely a Gateway Thor (OEM version of the Intel Advanced / ATX) so as @Cuttoon says, your looking for EDO simms to match what you've already got (2 x 8MB), though the board can support FPM simms as well (and possibly a mix thereof).

There are a few of the same Mitsubishi parts on the Bay - https://www.ebay.com/sch/Computers-Tablets-Ne … ipg=60&_fosrp=1 - or you could look for possible equivalent parts from other makes

As far as the cable goes, best I can suggest is what I used to do with 'sticky' 50 pin SCSI cables - use a very small screwdriver to gently lever the pieces apart (on the short edges, turn about, a little at a time) but ALWAYS support the outer piece with pressure when levering as they can be very brittle, even when new.

I agree with what's being said above.

In addition, you can try to loosen up the cables by pulling the cable out on one side only for a little bit, then pull it out from the opposite side a bit, then from the 1st side for another bit etc, until it comes loose.

And if you apply force, apply it at a straight angle. I'll fix up a quick paint to explain what I mean and update this reply.
EDIT:

Pull in the direction of the green fancy 3D arrows (lol).
Usually you'll need to wiggle them loose on one side at a time before you can pull them out.

I wasn't even thinking about taking a photo of the motherboard itself and only the RAM! Aside from knowing that the P5-120 is a Pentium of some kind, I wasn't aware of the motherboard's exact specs or chipset; though I do still have Gateway's documentation for the motherboard, it's half the country away in storage. I'm wondering how you guys figured that one out!

PC Hoarder Patrol wrote on 2022-04-28, 07:17:

There are a few of the same Mitsubishi parts on the Bay - https://www.ebay.com/sch/i.html?_nkw=MH2M325C … ipg=60&_fosrp=1 - or you could look for possible equivalent parts from other makes

HUGE thanks for this. I'm definitely going keep that table on hand.

In your opinion, would it be wise to get RAM from that electronics inventory store and not online? I already figure if I do I'm going to have to make sure the modules match those serials provided as well as take the P5-120 with me and test any modules before I buy them; they in fact welcomed me to do this, especially since they don't take refunds. They've got quite a selection, even if it's all kept in open bins (attached some photos so you can have an idea of what I mean; I don't know how delicate SIMM modules are compared to DDR modules of any generation).

Cuttoon wrote on 2022-04-28, 04:39:

If you don't want to risk damaging anything, leave the cable attached, use something like this to attach the USB bridge to the cable:

I should see if something like that is at the inventory store next time I'm there. Some of the stuff they have on hand genuinely surprised me. However, I can't recall offhand if the IDE cable has multiple connectors on the ribbon. I'll have to crack the case open again and double check.

Barring that, I'll try the suggestions everyone's provided to get the cable to come loose.

Trev-MUN wrote on 2022-04-28, 12:23:

I'm wondering how you guys figured that one out!

The Wiki page I linked to about the Intel Chipsets: Look at the line for "430FX Triton" - the chips are noted as "2x 82438FX" and those are in the picture. 😉

Trev-MUN wrote on 2022-04-28, 12:23:

In your opinion, would it be wise to get RAM from that electronics inventory store and not online? I already figure if I do I'm going to have to make sure the modules match those serials provided as well as take the P5-120 with me and test any modules before I buy them; they in fact welcomed me to do this, especially since they don't take refunds.

I would certainly take the machine to that shop and overstay the crap out of that welcome. 😀
In my experience, most 72 pin SIMMs work with most machines, but without a return policy and prices like that, that's a great offer.
That shelf looks gloriously chaotic, but if there's any sense to it, what you want should be in the "RAM 16 MB" box next to the "128D" sticker.
Just make sure you take a matched pair of each.
2 x 2 x 16 MB would already be "maxing it out" just for the sake of that.
Any DOS game for that machine will work if you just add two more at 8 MB, so 32 total. Getting 2 x 16 for 48 total is already plenty.
All of those take next to nothing in ebay auctions, but I'd love to have a shop like that, it's definitely more fun.

HDD issue: That "gender changer" thingy I attached a photo of is a generic electronics part that should be readily available on line or in specialized shops. The used hardware place, not so much.
But, I don't think you'll need that, just pull that cable. As others have pointed out, keep it slow/gradual and symmetrical, use some tool, if you have to.

Wanted to give an update a few months later; I hadn't been able to follow up on this data backup/memory upgrade project for a few months. I was able to get the IDE cable off of the hard drive following everyone's advice, but I ran into a different problem: I can't plug in BOTH the SATA/IDE to USB adapter and its molex power cable given the way the adapter's designed. It's not a problem for SATA drives because it's at a 90 degree angle to the adapter's USB cable, but that cable interferes with the molex power cable when using the 3.5" IDE port. So I had to look into getting one of those gender changers to plug into the IDE cable.

Yesterday I finally had a chance to go visit the store I mentioned selling all that memory and... I found out they're permanently closing this coming Saturday! Two years ago they had to move to their current location due to Amazon buying their previous building from the owner, and they're calling it quits now because the owner of their new location tripled the rent. D'oh!

I don't know if I missed out on a chance to buy RAM for my P5-120. There's a chance they might have had some and it got bought up already, but they still had a lot of random SIMM modules in stock. None of them had the EDO DIMM part numbers provided by PC Hoarder Patrol, though, in any size. Many weren't even by Toshiba, Mitsubushi, or Samsung. Saw a lot of Hynix, Texas Instruments, and a few other manufacturers.

They did have plenty of headers on hand, including something like what Cuttoon posted (but they were offset and wouldn't work in changing the gender of an IDE cable header) as well as some shrouded 2x20 pin headers that looks kind of like this:

I hadn't taken my computer with me at that point (precisely because I hadn't been able to back up the data first) but I did get to test this on one of the IDE cables for sale and it does seem to work as a gender changer if plugged in to the headers already on the cable a certain way.

I haven't tried it out yet, but it should work, right?

Er, you don't want EDO DIMM, you want EDO SIMM. That board only has (72p) SIMM slots, not (168p) DIMM slots.

The i430FX really isn't choosy when it comes to RAM, so long as the SIMMs work, the chip densities aren't too high (i.e. smaller than 64MB per SIMM basically) and you install in exact pairs you should be good. EDO is most common at 16MB size and gives better performance than FPM. Mixing FPM and EDO isn't recommended, but usually will work at FPM performance levels - but given your lack of experience and a certain level of haste with the shop closing on Saturday, just get four identical 16MB 72p SIMMs of any description. If they are identical, they should work and give you 64MB total. Worst-case they are FPM - but the performance difference is so small it's not worth worrying about in a build like this.

dionb wrote on 2022-07-13, 10:49:

Er, you don't want EDO DIMM, you want EDO SIMM. That board only has (72p) SIMM slots, not (168p) DIMM slots.

That was a Freudian slip. In the OP I did mention I identified the RAM as SIMM of some kind but didn't know anything more than that. I'm used to working with DDR RAM of different flavors, which is probably why I typed "DIMM" instead of "SIMM."

Anyway, I did find my old copy of MemTest86+ 5.01, I'll have time today to check and see if it'll work as a means of validating any RAM I buy from there.