Try going into the video settings of the game and lowering the resolution a bit.  That video card isnt a dinosaur but its a lower end one.  It wont be able to handle games with high end graphics that well.

With that type of Rig you should at hte very least put in a 8800gt or a Radeon 3850 or 3870 if ya prefer ATI.  This is at the very least.  With a nice card you'll run everything at highest res with a quad and 6gigs of ram.  Cards get pricey tho.  The ones I mentioned are "budget" higher end cards (150-200) but definatly not the best out there.