I did a couple of timedemo demo1's last night, at "high" detail with no antialiasing:

1280 x 1024 = 22.2 FPS with several visible pauses during playback.
1024 x 768 = 30.9 FPS with no pauses during playback.

Computer specs are 3.2ghz P4, ASUS motherboard, a gig of fast DDR RAM, and a GeForce FX 5900 with 256 megs of video RAM.

The game was playable at 1280 x 1024, just not comfortably. The lower frame rate was enough to give me a headache. But oh my goodness it looked gorgeous.