If you're using the latest guiminer you should have the option to use the phoenix miner; phatk kernel gives you some good improvements over the old poclbm kernel.
The non-technical version: create a new phoenix miner and use these flags:
-k phatk VECTORS BFI_INT FASTLOOP=false AGGRESSION=11
Of course if you're using this computer for anything other than mining you'll want to set AGGRESSION to something a bit more sane, like say 7
