I agree with the other two about you are just asking for too much bandwidth from main memory.
I would never make another monster machine because of this.
each video card has shared main memory and the worst card for this would be anything "onboard" video.
but if you really want to try this setup try setting all of your game video options to minimum and maybe reduce screen resolutions.
I was having problems with 3 copies of wow taking down my machine with the video set at maximum, each copy of wow was using 1.4G of ram (all max settings) , they use closer to 600-700M each now.

ps: I use the 8800 to run two copes of wow on one monitor, no problems at all.