Does ISBoxer utilize the GPU of an additional video card when the video is being displayed on the monitor connected to the extra card? Or is all rendering performed on the primary GPU?

I currently have 2 GTX 570s, and I'm pretty sure I'm hitting a memory bottleneck on my GPUs, as according to the EVGA utility, my vram is maxed at 1.2GB when running 5 wows.

I'm thinking of upgrading to a GTX 6XX series card, with 2+ GB memory, maybe even springing for a 4GB 670.

I have 6 monitors, currently I have them spread accross the 2 570s and the onboard Intel HD Graphics. I can't utilize the Nvidia surround feature, as the resolutions are not the same, so I need a second GPU to handle the the other 2 monitors. If I keep one of the 570s, would that give me better performance than using a POS $20 GPU? Or should I bite the bullet and get matching monitors so I can use the built in functionality of the new GPU?