Well assuming your computer has about a 300W PSU (which I'm sure it does, most factory computers come with lower power supplies) There is not much in store for you. the best performance that you can get from a 300W PSU graphics-wise is about an ATI Radeon HD5670 (or 6670, badge engineering). It will give you decent performance for about $70. For what you have though, I would recommend getting a new PSU (its not as hard as you would think, just remember where everything goes) and getting something around an ATI Radeon HD5770.
I'm not an ATI fanboy, its just that as Nvidia has higher performance, bugdet gaming is really where ATI has the realm. The 5770 (or 6770, badge engineering again) will allow you to play the best games on very high settings save for others that you may have to skimp on, like BF3. The 5770 will run you for about $110 or less (I got mine for $100). That will allow you to maybe pick up a better PSU
The things about PSU's though is that it is really the beating heart of the system, not the CPU. My one recommendation is that you DON'T BUY GENERIC BRANDS OF POWER SUPPLIES. Good brands include Ultra, CoolerMaster (most of them at least), Thermaltake, and Corsair. About 500W should do to power a 5770.
Here is a decent power supply at a good price (for now)http://www.tigerdirect.com/applications/SearchTools/item-details.asp?EdpNo=4362117&Sku=O261-2020
Also, here's a 5770 for a good pricehttp://www.tigerdirect.com/applications/SearchTools/item-details.asp?EdpNo=1412978&CatId=3669
And some 6770'shttp://www.tigerdirect.com/applications/SearchTools/item-details.asp?EdpNo=726760&CatId=7005http://www.tigerdirect.com/applications/SearchTools/item-details.asp?EdpNo=685692&CatId=7005