I'm running on an Athlon 3000+ @2,1 GHZ, 1 GB Ram, and a NVidia 6600GT 128 MB with full harbor traffic and it's running pretty stable even in harbors. I seldom have events where the FPS drops to below 15, but it's rare and only in harbors.
Check out
www.guru3d.com forums for the best driver for your card. At least with NVidia cards, the newest driver is rarely the best driver for your card. Currently I'm using a driver that's almost two years old, but it works best with my card.
Important: Tune down Anti Aliasing (AA) and turn off V-sync. Those are the frame rate killers. In case you're still not happy try using the low harbor traffic option and I bet you'll be ok.