The more memory the graphics card has, the frames it can buffer up and work on. I.e. while it’s display one frame, it can be working on the next few for the next time slices. This is more important when it comes to using high screen resolutions and anti-aliasing.
I.e., more memory will allow for more complex memory hogging graphics thus better graphics in games compared with a graphics card containing less memory. Note that you will need a high resolution monitor in order to handle high resolution graphics such as 1600x1200. This resolution may sound crazy, but I heard that the GF2 Ti card handles 1280x1024 without any problems in some games, although that’s without anti-aliasing.
64MB is recommended for today’s gaming, but I’d recommend going for the 128MB to future proof it a little.