I am successfully multiboxing with 2 ATI 4870 1gb cards (without crossfire). I use Innerspace to launch 2 of the wow's on the 2nd GPU and put the tank on the 1st GPU. Runs beautifully so far and I'm up to BC with my guys. I haven't gone to Dalarn yet with my 3 chars though...

I'm using Vista 64bit and 8 GB of RAM. I do not crash.

What is the temperature on your graphics card? Is it ever getting too hot and perhaps the reason for the crashes?