I don't mean to be a party pooper but that athlon x4 needs to go most games take advantage of 2 cores your pretty much stuck with athlon x2 performance processor wise, I would get at least a Phenom II x2 that has the 6 Mb l3 cache but then again the more cores the better but make sure it's at least a phenom II processor.
Graphics card wise I would not go to a high end card unless you buy a high end processor.
Right now the GTX570 is around $350. If you do that I would at least get a Phenom II x4 @3.7 Ghzhttp://www.newegg.com/Product/ProductList.aspx?Submit=ENE&N=100007709%20600094002%20600035073&IsNodeId=1&bop=And&Order=PRICE&PageSize=20
Something more in your price range is a GTX460 and a Phenom II x4 955 and both of these upgrades would be around $300.http://www.newegg.com/Product/Product.aspx?Item=N82E16819103808http://www.newegg.com/Product/Product.aspx?Item=N82E16814130562
4gig of ram you can get by with but you could throw in 4 more gig for @ $80.
This is a good overall rounded system for what you asked.