Does your computer reboot only when playing WoW or at any other time? Run a few resource heavy apps at the same time and see if you get the same rebooting. If not, then it could be as simple as a bad install of WoW.
If you do I would check your PSU with a multimeter. Have a look at the PSU Guide
to get a rough idea of what readings you should get, or refer to your PSU documentation.
If you have just upgraded your drivers then roll them back if possible, if not then I'd download older versions for the GFX manufacturers website.
Have you installed anything new recently? Try uninstalling any new programs and see if your problem goes away.
I can't imagine it to be a memory problem as you would normally get problems when the system is idle as well as when running programs.