In the GUIMiner...um...GUI, find the "Extra Flags" field (Just under where you entered your miner password). There are a ton of flags out the that can boost your hash rate some but I have found that this works best:
-v -w128 (typed just like that)
Clocked up to 975 core and down to 500 memory I get 300m/hash.
my sapphire 5830 pulls 300 m/h at 960/354 stock volts (guiminer on xp32 2.1 SDK / 11.6 driver. -f 15 -v -w 256 flags. havent tried higher clocks as its on a Delta 340 watt PSU, waiting for my Antec 550 trio RMA to come back. then Ill put the screws down.
there are many "sweet spots" - different work sizes and mem clocks can make a decent difference (10-15% maybe).