Dude it's not ordinary GALAXY OC Edition. The one which can be modded is the one which comes with 8 memory chips (6 working and 2 locked). It also comes with dual bios which is much more safer if u bust out one BIOS and u can recover with another.
The older 8800GTS(320MB) was 320-Bit whereas new G92 chip is 256-bit. So there might be some difference, but the newer 512MB 8800GTS performs better, that u all already know.
3D mark always give ATi more points than nVidia so don't trust 3D mark scores, u can keep them as reference. Actual performance comes in gaming where eye candy meets FPS.
After all this, i don't think that 9600GSO performs better than ur 320MB edition, i m not sure, search for some comparison chart.