My issue sounds exactly the same as yours.

I was originally thinking it was just a crappy cpu, i'm still using a athlon xp 3800+x2 64bit.

But since your clearly superior machine is having the same problem i'm going to go with something else although i'm not exactly sure what. The only thing that we have similar is using the radeon hd cards. Although mine is a 3870x2, they still use the same driver. As of yet no driver updates from ATI yet so i'm not sure if many people are having the same problem or not. If you manage to find working drivers it would be awesome if you could post a link (even if they're tweaks, not fussed)

It's probably a driver problem (on ATI's side). This is the main reason I'm an nVidia person: performance-wise their cards are comparable and usually equal, but nVidia cards get far more attention on the driver side.

run memory check utilities.

He said MW2 is the only thing that's causing problems, so it can't be a hardware problem (unless that game uses some part of the computer that no other program does).

that's not true, not all games work the same way when taking resources from the computer itself, there could be a certain block that's bad but not accessed often or being checked off as a bad buffer by the system, if that part of the memory is not touched often, system stability wouldn't be an issue, but if you get there, it will blow up regardless. though tbh I think it sounds more like a power supply issue, if you have a volt meter around the house, i'd check the power outputs of each connection to make sure they are all at the right voltage, this is a PC, not a console, you can't apply standard logic to all problems.

Actually most games and applications don't refer to an actual memory address. Usually a block of memory is set aside if a program asks for it, but rarely does the program ask for a specific memory address to be set aside. This would be extremely inefficient and erroneous, as that block could be in use by some other program, and overwriting it would lead to unexpected behaviour. Every time the program's run, it uses different memory locations.

And I still doubt it's a hardware issue, especially since he claims that BioShock has no problems running.

I found a short term fix for it, rather than running the game in 1280x1024 as i usually do, i ran it in 1024x768 worked fine for about 2 hours straight. Dont think theres any problem with hardware the cpu is working its nuts off but it doesnt lag or jump around at all

It seems like your system was actually really close to the minimum requirements. He should be able to run the game without any problems.