Is my 8800gt or my motherboard causing this BSOD? (nvlddmkm.sys)

I just built a new PC and used an 8800gt that I've had for a couple months. I've got everything running correctly now except for a BSOD that keeps popping up when I play WoW or AoC for a while. It's happened with three different driver versions (175.19, 177.79, 177.83) but seems to have become less frequent while using 175.19. I was able to play AoC for about an hour or so before anything happened today.

The BSOD cites nvlddmkm.sys as the problem but nothing I've done has solved it. I've tried uninstalling the drivers and reinstalling after cleaning up with Driver Sweeper but that hasn't helped. I heard that underclocking my memory could help but dropping it to 667mhz didn't do anything. I've tried using MemTest but it didn't return any errors after an hour. I'm currently running it again. I'm using the latest BIOS and chipset drivers on my motherboard and the only overclocking in my system is my video card which was factory overclocked. My system is:

Intel e8400 @ 3.0ghz
ASUS P5N-D nVidia 750i SLI motherboard
Zotac 8800gt 512mb
4gb (4x1gb) Patriot Extreme Performance 800mhz
Raidmax 500w PSU
320gb hard drive

Any help would be greatly appreciated.

Woops, I can't believe I forgot my OS. I'm running Vista 64bit. I haven't had any problems while on the internet or things of that nature, only while playing WoW or AoC. I'm starting to get desperate and may have to try reformatting my hard drive and reinstalling vista if this continues. Could that help in any way?