Re: 6600 256MB vs. 6600GT 128MB
If it isn't out of your budget the 6800GS is a much more capable card.
I did not suggest it because you did not mention it.
The 6800GS has 256 meg of ram or 128 meg?
If it's 256 meg of ram, snatch that up. Even if it doesn't unlock it's a better card than the 6600GT. The 6600GT isn't a bad card at all. It's just the 6800GS is better.
The 6800GS has a 256bit memory controller. The 6600GT has a 128bit memory controller.
To unlock, you load up RivaTuner, select a few menu options (might need a reboot) and it will either give you a speed increase with no degradation in graphics quality or it may glitch... Glitch is just that, you see a graphic "glitch" on the screen. If that happens, you merely unselect the options you selected (might need to reboot) and it's all good.
But, look around for a better message thread about unlocking using RivaTuner. I've never done it.