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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...
 
I dont have any problems playing COD 4 in bootcamp. I play it on high settings with a 1024 x 768 screen resolution.
 
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