memadmax :
If ur not crashing on anything else then memtest will do nothing.
Are you running addons with wow by any chance?
If you are, disable them and try again.
I run AMD vid cards and have no problems with wow unless I have crossfire turned on. make sure that is off. Yea, AMD drivers are weird sometimes.
Bad ram could theoretically work 99% of the time, especially if you've got 8GB to work with. You'll only crash when you try to use a bad hunk of memory; so it's not impossible that only wow crashes, especially if he's running the 64bit launcher (where almost every other game is 32bit, and restricted to a smaller memory footprint).
If disabling addons fixes the issue, that points even more at a hardware issue. Wow Addons have a rather limited API, and while it's conceivable that a bad add-on could crash WoW, they do not (or rather SHOULD not) have the sort of access that could crash the entire PC. So if the addons cause the crashing, it's likely due to increased CPU/RAM usage.
@OP
Though, one thing that hasn't been brought up... Have you tried running a few different anti-virus tools? If you've got some poorly coded mal-ware that targets WoW (trying to steal your account or whatever), then it's conceivable that it is crashing your PC. I would say this is a stretch, but it's worth investigating.