I am doing this and use two different profiles for pure mining and mining while gaming respectively.

I'm using the phoenix miner with the phatk kernel and setting the AGGRESSION parameter to 3 I can fluently play any of my games on my HD6850 and still get about 100-110 MHashes/s.
For pure mining I set AGGRESSION to 16 which after trying differing settings yields the most (about 262) MHashes/s for me.

As for CPU mining, I let the ufasoft miner run at 100% and low priority and got no problems so far (and an additional 13 MHashes/s while my PC is otherwise idle).