In the future, more and more games will become texture intensive...and will load massive textures onto your GPU. This is where having more video memory will make your games smoother. Remember...more physical or video memory does not equal higher frames per second....it equals smoother frames per second. This is a common misconception. Few games that currently have massive GPU memory texture offloading currently are Doom 3, Quake 4, Flight Simulator X, and Age of Empires 3.
Having 128MB would not hurt...infact in terms of frames per second it would make no difference....its juts that in games that have massive/high res textures...the game would stutter due to harddrive texture scratching. The actual speed of the game depends on the amount of pixel pipelines (Dx9 cards only), amount of stream processors (Dx10 cards only), and the core/memory clock speeds.
Another reason other than gaming where having a higher GPU memory would be specific to OS X itslef. Quartz Extreme takes advantage of GPU memory, and uses it efficiently, so your animations in OS X will be more smoother on 256MB vs 128MB....however the difference will only become apparent when you have lots of windows/apps running...and try to do something like Expose.