Your processor, at least try to grab an i5-2500k.
Maybe add on 2 more gigs of ram.
And of course, upgrade the graphics card. If you have 200 bucks, I would recommend an ATI Radeon 6850 or 6870 for your card.
I'm not really a fan of nvidia. But if you like nvidia a lot, upgrade your Geforce 250 GTS to a Geforce 450 GTS.