A Pentium 4 isn't all that great, but an 8400GS is pretty bad, it came out in 2007. However, you do need a PCI card since your computer doesn't have PCIe or AGP, so you are pretty limited. http://www.newegg.com/Product/Product.aspx?Item=N82E16814500228
is probably the best card availible. You probably don't need to upgrade the PSU, PCI cards are pretty low on power draw. The 520 has a max power draw of 30w.