Currently, the most demanding test with regards to complexity and texture is Game Test 4 (Mother Nature) from the benchmark program 3D Mark 2003.

As was the case with Unreal Tournament, there are no differences in standard performance. Because the benchmark uses very large textures, you can already see a difference with 4x FSAA. However, this difference looks larger than it actually is at just about two fps, due to the smaller scale in the table. With 6x FSAA, the difference increases to nearly three fps. Again, the differences between AGP 4x and AGP 8x are easy to see.

To reiterate, those high resolutions are the only cases where you can expect a difference between 128 MB and 256 MB cards. Under "normal" conditions, there's absolutely no difference (see: NVIDIA GeForceFX 5900 Ultra: The Way FX is Meant to be Played!! ).