Jaymem,
A 2 GHz cpu is not too shaby. Is it an Intel or AMD cpu? Also 512MB of memory is ok. What is the video card that you have? Also, what games are performing poorly and have you tried to turn down the resolution, some of the graphic settings, etc.?
If I had to guess, I would say it is your video card which is underpowered. For your machine, a Radeon 9800Pro would do nicely.