non-OC is the only way. Mine is a totally shitty OCer (835mhz is max stable without going crazy with voltage) and does 345MH/s.
Go to the mining software page, download the upgraded phatk kernel: http://forum.bitcoin.org/index.php?topic=25860.0
Use these flags: -k phatk VECTORS BFI_INT AGGRESSION=13 FASTLOOP=false WORKSIZE=256
Adjust aggression if you're not running a dedicated box.
Overclock without raising voltage. I use AMD GPU clock tool. Set ram to 300. GPU to 800+ (depends on what it can handle).
Enjoy extra MH