There was an ExtremeTech review of ATI 9800 cards not long ago comparing the 128 MB model with the 256 MB model. The benchmarks of all games gave results that were within 1 fps of each other across the board, even when looking at Q3 running over 200 fps (= 1/2 % increase in speed with double the RAM).
Increasing the RAM only improves speed up to a point and then beyond that point the extra RAM is just redundant. This point is dependent upon the requirements of each individual game.
Personally: I just doubled my system RAM to 1GB and haven't really noticed much improvement in the performance of my computer, including the fps of my games. In fact the D3alpha runs slightly slower, even after all the tweaking to utilise the extra memory.