VRAM isn't as important as how fast the GPU can push data through the card and into other parts of the system. High clock and memory speeds (500/1000 MHz on the AGP GDDR-3 model) and eight data pipelines give it the edge over the 9800 Pro. The 9800 Pro may use 256MB of VRAM, but it uses the DDR-II standard which is not GPU specific. The 6600GT uses 128MB of GDDR-3 RAM which provides for better power management and higher memory speeds than DDR-2.

The ironic thing is that GDDR-3 was developed by Ati, but they didn't start using it until the X800 came out. Meanwhile, Nvidia has been using it since the 5700 Ultra.