128 MB could make a difference with FSAA enabled, theoretically (again). However, NVIDIA's GeForce3 no longer uses the memory-hungry super sampling technique, but the more efficient multisampling, which requires a lot less memory than super sampling.
Nonetheless, the extra RAM should help out here. Here's why: 64 MB cards only support 4x FSAA up to 1280x1024 because of memory constraints. A 128 MB board, however, can reach 1600x1200.
Once again, the 128 MB cards show no real gain. While they do allow 4x FSAA at 1600x1200, this is only of academic interest, considering that the framerate remains somewhere in the 20's.