The 128 MB core is clocked the same as the 64 MB version, but the 64MB has about 15MGhz (something minimal like this) faster memory. This makes this 64 score better on the THG benchmarks. However you can overclock the memory on the 128 to the same as the 64 (using a program called Rivatuner) and the performance should be about the same. In newer games like UT2k3 and the long antcipated Doom3, you should see at least more consistent if not better frame rates. If you can afford the 128 version, I'd just get the 9500 Pro. For a little bit more you can own the ti4400. If not deffinetly the 128.
If its not broken, you're not trying hard enough.