The general consensus is that the GTs are better - the memory size doesn't matter too much:
& click here
A quote: "A 6600GT kicks ____ on a stock 6600. Forget how much RAM it's got, the standard card is simply no competitor."
Some benchmarks (6600s & 6600GTs included) that may be of interest: click here (for AGP) & click here (for PCI-E).
Purchase links, if required (use the Filter at left to narrow down the search according to your budget/interface/etc):
- 6600GTs (128 & 256Mb/PCI-E & AGP) sorted by ascending prices click here.
- 6600GTs (256Mb ONLY/PCI-E & AGP) sorted by ascending prices click here.
- 6600GTs from eBay (128 & 256/ PCI-E &AGP) sorted by ascending prices: click here.
Good luck with the selection/purchase & enjoy the gaming : ) G