Couple of things that might help - only played with the card for a couple of hours but it seems that the - XP driver suks - Vista driver will be much more stable ( and no more crashes in ATItool etc. ). Higher Vcore appears to hurt shader clock - keep the shader clocks lower and you might be able to get the GPU clocks up.
Curreently running @ 890/1850/2400 voltages 1.25V (1.3V 3D loaded ) / 2.08V
Not possible for me in XP
Shaders I was able to run 2300mhz with default Vcore. Might be a fluke - but you guys might find similar behaviors