The only part of your system that's lacking is the graphics. If you drop about 150 into a new card, you could see some nice improvement when you do graphically intense stuff. Otherwise, your system is fine as is.
I'd recommend for example,
EVGA SuperClocked 01G-P3-1461-KR GeForce GTX 560 (Fermi) 1GB 256-bit GDDR5 PCI Express 2.0 x16 HDCPhttp://www.newegg.com/Product/Product.aspx?Item=N82E16814130661&name=Desktop-Graphics-Cards