Well, the 64 is better than the 32 but costs more (funny how it works that way). I've run the 32 and 64 side by side. The extra texture memory DOES make a big difference. I dont mean super high res either. I mean 1024 x 768 x 32 x max quality. For example: UT is smoother on the 64 especially in large maps like Deck16. Q3 takes a very small FPS hit with TC disabled. You will see a larger hit on the 32 meg card. New games like Serious Sam, Giants, B&W, etc have very large textures, and that will only get worse (better actually). The smoothness difference doesn't show up all that much in benchmarks, but does in actual gameplay.
If you are getting the card as a stop gap until the next gen cards are out, go cheap and get a 32 meg. If you plan on keeping it for awhile, get the 64 meg version. This goes with any card, not just Radeon.