Same thing I've done since I started mining anything...
Mine with total cores -1. Full speed GPU, good speed CPU mining.
On primecoin I had it at setgenerate true -1 (all cores) but that was sucking the power out of my gpu's for litecoin.
I changed it to setgenerate true 7, leaving 1 free, and got the much improved performance, albeit with the 12 percent litecoin loss.
Set the cpu affinity of CGMiner to use 1 core, and set it's priority to High (you can do this from the task manager or through a bat file by placing "start /affinity 1 /high" before CGMiner).