Nothing seriously high...
Dual core (or better) processor
At least 2GB Ram
ATI 5770 or Nvidia 8800/9800/250 (or better) video.
If you want to turn everything up to max with shadows and everything, I'd recommend a ATI 5850 or better, or at least a GTX460 Fermi Nvidia card...
Personally I had an Nvidia 9800GTX+ and it did well enough with the sliders about 3/4 but I 'upgraded' to an ATI 5830 which is a bit better than the 9800GTX was... regardless as long as I leave shadows off I run 60FPS in almost every circumstance. If I turn shadows on, I can run 2048 detailed shadow textures and drop down to about 50FPS every 'once in a while' but I personally find the shadows to be annoying so I leave them off for the performance increase. At least with the 5830 I can crank the sliders to full.
This is at 1920X1200 resolution running 2048 textures (with the hi-res pack downloaded and installed) and a Core 2 Duo processor (E7400) slightly overclocked to 3.15GHZ... and 4GB of DDR3 ram, albeit the slower stuff.