Re: 6600 256MB vs. 6600GT 128MB
Few things to remember.
You need a power supply that cranks out a lot of AMPS on the +12v rail.
6600GT AGP needs a power lead plugged into the card. Prefereably nothing else sharing the lead. Don't know if your current card uses one or not...
If you have performance issues, look into uninstalling and reinstalling the motherboard GART driver for the AGP bus. It's easy and is often the solution to performance issues.