That hasn't been figured out yet, but will be dependent on what CPU and what video card you're running. Is that a screen shot of your rig in your sig? The Q9400 and 8800GTS?
Personally, I would run the CPU at 75% leaving one full core free for the GPU. Once your RAC stabilizes, you can slowly increase the the CPU usage to see if it's more helpful or harmful to your RAC.