My 5830s ($100 each) are pushing 260 MH/s at 875 MHz core frequency.
What SDK version do you use? Had a similar speed (266 @ 900 MHz) but then dropped to 248 after some experiments with SDK version and I can't figure out the cause. Using Win 7 SP1 x64, SDK 2.4 currently, and phoenix miner with phatk kernel on that particular machine.