you should be getting more fps. my other pc with a amd x2 3800+ oc @ 2.9ghz 2gig of ram and a 9600gso oc @ 715/1040 gets around 30-40fps with all settings on high 2aa and at 1280x768.
i think you should oc your cpu my amd x2 3800, i drove from stock 2.0ghz to 2.9ghz which gave my fps a huge boost.