You will need to talk to the GTX 295 king.

I myself run 8 threads of WCG on ea of my I7 920, and 4 threads on my q9550...adding in a Xeon W3520 tonight or tomorrow as well. Also I am running 1-2 GTX 275's per rigs. You will also want to bump up your shaders from stock if you havent already, keep in mind with Folding you cant run shaders as high as you can on other stuff. I am not sure what the guys are running the 295's @.

Hopefully the GTX 295 king will chime in soon to help you out more