64mb of ram is almost overkill itself. Dont bother with the 128, its all marketing.
For some reason, people seem to equate amount of memory to performance. All my friends ask me, how much ram do you have on your video card? I say 64, he says 64. So he thinks we're equal. Never mind the fact I'm using a Ti 500 and he's using a GF2 MX200.
RAM is for your frame buffer and texture memory only. If everything fits in 64, or even 32, (as most games will for the next two years), there will be absolutely no reason to buy more memory.
You dont need 128mb to do FSAA. 128mb would help if you do 1600x1200 4XAA, because thats a lot of samples. But you'd get crappy frame rates anyway, so why bother? At 1024x768 with even 4x AA, the 64mb will be absolutely faster. Only when you stretch into ridiculous already unplayable resolutions, the 128mb will not be FASTER, it will just stutter less.
Faster memory = more fps. 500 is 1.13 times faster than 444. And that means 13% more fps, if you have a fast enough CPU. Its that simple.
Dont waste your money on the 128 unless you plan to keep it for 4 years!