my pc specs are a little higher than yours and i have to run EVO on low display settings and 2 AA to stop the frezzers and even then i still get stuttering on some tracks when passing the pit area.
4500 @ 2.2
8600 512mb
4 GB ram
That's REALLY odd.. I have a laptop with dual 2.0ghz, 8600M gs and 2gb ram and can play max with 30 fps on Brans and up to 100 on most others. Can put AA up but don't because of tracks with lots of trees.