Everything has potential for moderate gaming, depending on how you define "potential" and "moderate".

Anything that was used in an office would probably need some kind of graphics card installed; that's pretty much all that really matters. If you want to run something specific, then look at the system requirements.

I would use them for playing games from early 2000's. They would def need discrete graphics maybe GT 730 2Gb cards, perhaps even 750 but would that be bottle-necked by the older CPUs? Other than that i would upgrade fans and possibly the PSU if needed as well as getting new HDDs.

Personally I think the whole issue of "bottlenecking" is overstated. Buy the card you can find at the price you can afford and that supports the games you want to play, and forget about whether it matches the processor according to some arbitrary metric.

Not sure why you would need new HDDs, unless you wanted to go SSD, but if you're that concerned about performance, then you shouldn't be messing with old machines in the first place.

Boss had me take out the hard drives and destroy them. I explained I could just format them but he insisted on physical destruction since he has no idea how technology works and he's super paranoid. Ssd drives would be cool but would they work on Windows XP? I could always use Windows 7 as well but I'm more inclined towards XP just for the purposes of running older games

A quick google search can find ya plenty of guides on how to set up xp to run on an ssd. Id link you a couple but at work on my phone. A 750ti would be a good match I think for a Core2.

http://wp.xin.at/archives/829
scroll down to part 5 - that utility lets you run TRIM on WinXP.

An alternate way to mitigate the issue for any OS without TRIM support is to overprovision (underprovision?) it.
First TRIM the whole drive (running a secure erase command will accomplish this), and then leave some of it unpartitioned, like about 25% or so. If 25% of the disk's logical addresses are never used by the OS, they will never be flagged as pointing to valid data, which means the drive will always have at least that amount of unallocated space in the physical storage. That means the drive's garbage collection routines will always have that amount of slack space to work with and you won't get the massive write amplification that results from a drive whose firmware thinks it's full.

I got my first SSD from eBay about 6 months ago or whenever it was. I've been running XP on it on a system that I use most days, but admittedly not heavily. When I got the drive, the SMART status said it had 99% life left, and that's still what it said when I checked it a week ago.

I managed to rescue an Optiplex 755 from my workplace under similar conditions... The plan was to turn it into a high-end Win XP machine. First thing I did was to replace the lowly Pentium Dual-Core E2160 with a much more capable Core 2 Duo E8500. This was a very easy and cheap upgrade, as the chips are dirt cheap on eBay. I also wanted a better video card, but my machine is the desktop model, so I'm limited to low-profile cards (I know there's a right-angle adapter for larger cards, but I don't have it)... ended up going with a Radeon R7 240. I was able to get this all working, but was getting lousy performance from the Radeon under XP. I couldn't even run Oblivion at a playable framerate, and I couldn't figure out what was going on.

Eventually I decided to try wiping the machine completely and installing Windows 7... and suddenly everything was running great. I can crank up all the graphical options on Oblivion and it runs smooth as silk. So I guess ATI's Windows XP drivers just totally suck balls when paired with newer cards? I don't know, but I've left it as a Win 7 machine. Stalker, Unreal Tournament, Ultra Street Fighter 4... they all run great. So yeah, moderate gaming is absolutely possible with these kinds of machines.

I definitely recommend swapping in a faster processor (Your boards might support Core 2 Quad, but you might get better performance using a Duo with a higher clock speed, as single-core speed tends to be more important with older stuff), but if you want to go with Win XP, I'd recommend getting a more period-appropriate card rather than something actually modern like most people have recommended here.

--Zero

I ended up ordering a couple of used Core 2 Duo e8600's to replace the C2D E4500 in the Opti 330 and Dual Core E2200 in the 360. The Core 2 Quads seemed like a good choice but they're rated at higher TDP (95W I believe) and I wouldn't wanna push the heat on Dell boards, also they're not cheap at around $60 US for used ones. The AMD machine I discovered is packing an Athlon 64 X2 5000 which isn't bad at all. As far as the 360 and 330 Optiplex I'm limited by the small size of the 330 so as far as a GPU I may pick up a low profile 750 Ti for the Desktop case and another full-size card for the Mid-tower 360, maybe a R9 270? Just to compare between the two set-ups since both PCs are only about a year apart in age.