I use the following flags on my 6950. I find it makes my computer smooth and hashrate high. In fact, the Mhash/s rate is higher than without flags (337 vs 284). don't ask me why.
My flags are:-v -w128 -f200
-v to enable vector support
-w128 has something to do with optimization.-f200 is the secret for smoothness.
It limits the frames (or framerate?) to something rather. It doesn't seem to affect the hashrate in idle!
I don't even notice that GUIminer is on while watching vids and doing desktop stuff. Just awesome! http://image.bayimg.com/4a892ebc06d8db506895fd6fc9ee8f510a171504.jpg
I set the CPU affinity to my last cpu-core only, since that's the one that is the least used by non-multithreaded applications.