Athlon 64 3000+, 2Gb Ram (4x512) and a vanilla 6800.
Have only played single player so far (only got it this weekend), but so far it runs great. Load times are slow, but once loaded it is stutter-free.
Running on the auto-detected specs - 1024 and 2xAA (anti-aliasing, since you asked).
And last night I was running it with emule running in the background (forgot to turn it off) - didn't even notice.
Clearly a very well-programmed game. Hats off to Relic.