Well it depends on your budget. With a resonable budget I say go with an i5, 4GB of DDR3, and a P55 board that supports SLI. If you're budget is only like $200 though then you can go AMD and get an Phenom II 550 BE and a decent board while keeping your RAM and everything else. Of course I recommend overclocking that 550BE so it would be good to get a cooler if you had the cash for it
As shumbhan suggested, you may want to try overclocking your CPU a bit for now.