I know this thread is a month old now, but I just installed my new AGP XFX 6600GT with 256mb so I thought I'd chime in.
I was able to get to 582/1160 with stock AGP voltage and cooling. I bumped my AGP voltage up to 1.65, and was able to get the memory to 1170, but then I couldn't get the core to 580 anymore. Since there was no true gain, I brought the voltage back to stock.
I just installed 3dmark05 and ran the tests at the default settings. I got 4229.