i have the same specs, 2.2, 2 gb, using xp sp2, 128 8600m gt.
I play to external monitor, at 1440x900 med to high graphics, no AA rather trilinear.
Single player couldnt be more smooth. Multiplayer i avg 50 fps. Sometimes as much as 120 (when its really cold inside my room, and not much action going on in game)
i do have the previously mentioned laptopvideo2go drivers for it.
I'm playing UT4 on OS X.
I just checked, and the bottom res of 640* also stutters, so I guess it's hard drive related rather than graphics card. Hmm...