Since you just do a little gaming, you may be able to get away with just a hard drive and video card upgrade, and some added RAM.
You could upgrade to a PCIe video card, such as a radeon 4670, 4770, or 5770. They are all fairly reasonable HD video cards. This will likely also require a PSU upgrade.
An upgrade to at least 1GB of RAM would be helpful, specially if running Vista or Windows 7.
For ~ $55, you can get a 500GB Samsung spinpoint F3 hard drive. It's one of the quickest 7200 rpm hard drives available.