I just bought a PCI based GPU (video card) for a system only slightly stronger than yours. I bought a Radeon 9250 with 128MB of memory that I am very happy with. However, there is also a 256MB version. Link is below.
Your current power supply should be fine. You really don't have many PCI video card options, but this is a solid card and should do you well.http://www.newegg.com/Product/Product.aspx?Item=N82E16814161184