One thing I have noticed while playing 5 wows at once is that the nVidia driver will eventually go insane and enter a restarting loop (this is in vista - i don't think XP had the ability to recover from a graphics driver lockup) at which point i can only close up shop and restart the machine.

At first I tried a number of different nVidia driver versions, which didn't help. Most recently I downloaded the nVidia overclocking tool and turned the gfx card fan to 100% and underclocked my graphics card slightly. This seems to help a lot and i can normally run sessions for hours and hours without trouble, but every now and then it still goes ape on me.

I think my next step would be to try a water cooler, or maybe a different brand of video card (though BFG isn't known to be junk afaik, but it was one of the kind that came pre-overclocked, so that was likely the issue)