For ATX boards, I use whatever ATX case I have on hand for the most part.

You really don't have to worry if the old ATX boards will mount in new ATX cases.

For AT, I guess it depends a bit more on the case as to whether or not it has the correct places to mount an AT motherboard.
The only real problem is that you will have to either make an AT I/O panel cover for the case of buy a pre-made I/O panel that just has the hole for the DIN-5 keyboard port. Well.. you don't HAVE to have one, but it makes it look nicer than just not having the I/O panel.

So I could drive down to my nearest MicroCenter and pick up any full-sized ATX case and my old Pentium II motherboard should mount up in the pre-drilled holes/standoffs? What about the switch wiring...will that work as well?

They all seem to have a space for a power supply that mounts on the bottom. Will my older ATX power supplies fit in that area as well?

For instance, I was looking at a $49 Thermaltake V100.

Yep, any ATX should work.

As for the power supply, is it a good brand? I tend to not trust old power supplies as most of them from back then were junk. And if they have been sitting for quite a while, if the capacitors weren't already bad when you stopped using the power supply, they have a good chance of being bad now.. if it is an old, cheapo power supply.

Mounting the power supply in the bottom of the case is of course a newer thing. The only thing you have to watch out for is if the motherboard cable(s) from the power supply is/are long enough.

ATX has not been tweaked, just extended. It would not be a standard otherwise. New cases are great as the usually come with 120/140mm case fans, including side panel fan support, which is great for keeping your Voodoo2s cool. USB can be a pain, but you can easily buy converters for the front panel connectors. Modernn ATX power supplies will generally work well too unless you need -5v or something weird.

Power Supply I have is a new old stock 300w Antec from around 2003 or so (no SATA on it). I was hoping it'd be ok so that I don't have to buy a new one. Trying to keep this limited to what I have on hand, other than buying a case and an IDE to Compact Flash adapter (for the hard disk). Perhaps a SATA to IDE bridge adapter if my old IDE CDROM is dead.

I was also looking at the DIY open chassis brackets as well. That way I could easily swap stuff around, and it's not like this will be a daily driver where I'll have to worry about dust or sticking it under a desk or something.

_tk wrote on 2020-01-11, 00:11:

Power Supply I have is a new old stock 300w Antec from around 2003 or so (no SATA on it). I was hoping it'd be ok so that I don't have to buy a new one. Trying to keep this limited to what I have on hand, other than buying a case and an IDE to Compact Flash adapter (for the hard disk). Perhaps a SATA to IDE bridge adapter if my old IDE CDROM is dead.

I was also looking at the DIY open chassis brackets as well. That way I could easily swap stuff around, and it's not like this will be a daily driver where I'll have to worry about dust or sticking it under a desk or something.

Open it and check the caps. Some of their models were being built with Fuhjyyu caps which are extremely prone to failure. The designs are usually fine otherwise and give good service upon being refurbished.

Yeah, I'll probably open it up just to check. It's an Antec SL300S and I'm 100% positive it has never been used.

I will say that it's fun going down this road again. I'm honestly surprised that there are decent "solutions" still available for these older machines.

Yeah a new old stock PSU is just an old and likely dangerous PSU. I would rather take my chances with one that has been used and is still running after all this time than surprise 20 year old caps with 240v.

I'm in the US, so I'd only be surprising it with 120v!

I get what you're saying though.

2 problems you may run into with new PSU's is I'm noticing some are dropping the 20 +4 ATX connector breakaway. so just keep an eye for that.
Also if the PSU mounts on the bottom of the case sometimes the cable isn't long enough to reach the motherboard connector.

I'm noticing some are dropping the 20 +4 ATX connector breakaway.

Can be fixed with ATX extender cable.