aye. i have an nvidia rig too. i get the same troubles. as always,make sure your drivers are up to snuff.
if i'm not mistaken,with nvidia cards,the number you need to note is the one second the far left,if that makes sense.
for games,you want something at least a 6 there,preferably like an 8 or 9. anything else (5 or below) is only recommended for desktop applications,not 3-D gaming.
that is why i have conceded to playing the game on medium settings, with a few custom options to taste. (8400 gs card,albeit non-onboard) also,try running your 3D settings under nvidia control panel set to "application controlled". seems to help a bit as well.
works much better than trying to get blood from a rock on high settings,especially in high traffic areas w/ choppy sea conditions.
also there's this:
http://www.nhancer.com/?dat=d_start
which may or may not help. i tried it once,couldn't get the hang of how to use it. it's not exactly designed for graphics enhancing beginners like me,but that's just my opinion.