The card should have a tiny switch on it which gives you two options --- default or overclocked bios.
The switch is tiny, but once I found it, I get steady 590 Mhash/s on the overclocked bios bank using GUIMiner (I believe overclocked is bank #2)
It does have a switch yes. But I couldn't see any difference with the switch in position 2, aside from it being more temperamental.. (The driver kept locking up)
My sweet spot now is just over 590 MH/s, at 1100MHz. I might be able to get 620 @1150 once I work out the ideal voltage, but it's tricky.