It won't be a big improvement. The 6600s are hamstrung by the 128-bit memory controller, not the 128MB of RAM -- the 6600GT has less memory bandwidth than a 9800Pro. Digit-life tested a 256MB 6800NU against the 128MB version and showed no real difference in performance (a few things were faster at 1600x1200 with AA/AF, but both were very slow at those settings); the 6600GT is likely not going to be any better.