On an I7 (even a Sandy Bridge) you may need to leave a full CORE not thread if you are running dual 5870s. On my machine with a 5970 and GTX 570 it takes 26% of the processor. A FULL CORE so even with a V8 you need to set use 75% of processors.

As Maxwell says, the object is for the GPU to be at 97% load full time ... anything less and you are losing credits that the CPU crunching can't touch!

I actually do it another way. Install the program ProLasso (Process Lasso) and using that you can set priority and affinity for all processes. I setup threads 6 and 7 for the Moo GPU programs (dnetcxxxxxx) and set the priority to HIGH. Then for any CPU tasks I set them for threads 0-6 with normal priority. On the 5970/GTX 570 I do the above but I set the primegrid wu to threads 0-6 with HIGH priority.

The benefit to totally isolating the GPU core is that the GPU doesn't have to interrupt a lower priority CPU task to get serviced. Even this minor delay causes a loss of GPU processing.