I think it depends more on the card, as I wouldn't bother with 128MB on a R200/GF4 or lower GPU if it costs significantly more. By the time you would need 128MB for any game, you'll surely need a faster card to take advantage of it. Also, cards with the same GPU tend to have slower clocked RAM which will have a more tangible impact on performance than an extra 64MB of slower RAM.
The only current "gaming" application that takes advantage of more than 64MB is 3DMark2K3, which states in its whitepapers that some tests can use up to ~124MB of video RAM. I'd go with more system RAM for current games (512MB), as real world games make better use of system RAM for textures than > 64MB of video RAM. 3DMark2K3 performs better using video RAM than system RAM, but in reality system RAM > swap file which would be the first thing impacted by large textures. But then again, 3DMark2K3 would probably run better with a molex adapter from your PSU and the card plugged into a monitor.
Chiz