I'm running:

Pentium Extreme Edition 3.73ghz
4gb RAM
2 x Crossfired X1900's 512mb

1900x1200 resolution on 24" widescreen LCD
No AA - I don't feel I need to use it while running at that resolution.
Everything on max except shadows which are turned off
Huge units

Don't have FRAPS installed so can't tell FPS, but framerate is fast to the eye, which is all that matters. It does slow down a bit in large sieges.