NFSU2 is way more demanding than any of the specs on the box say it is. My specs:
P4 2.8Ghz,
1Gb DDR RAM,
FX5900 XT TDH 128Mb,
and it still judders slightly in certain places on the map, namely the hilly area in the far north, and the industrial area in the far south.
I would suggest doing the following:
Turn all graphics to the lowest possible level. Then slowly, turn them up, one by one. Do this in order of what you would consider greater importance (depends how fussy you are about your graphics - I couldn't give a monkeys about anti-aliasing, so I leave it off). I did this until I found the graphics smooth, then I fiddled a bit more until they were balanced enough for playing. The only way I could ever solve my jerking in the areas mentioned above, was to drop the resolution to 800x600 (normally 1024x768). Looked terrible, so I learn to play with it.