It depends on the card.
I have 2 graphics cards at the mo
A Geforce 5900xt 128mb. Core speed 390mhz.
A Radeon 9600 256mb. Core speed 325mhz.
Guess which ones in the draw twiddling it's thumbs?
Its down to rendering speed and the geforce is simply far faster at it. The radeon's gpu cannot render aswell so having more texture memory is a waste of time.
Benchmarks show that most cards with 256mb ram dont perform much better than the same card with only 128mb. Infact some cards run worse with more RAM.
Its the same with PC's in general, once u have enough system RAM that the cpu needs to reach it's max putting more in wont make a difference.
I'd have 3gb in mine if it did!