How do i know whether to change gpu or cpu

Hey. I have a question whether to upgrade my computer's GPU or CPU. I just need it for light gaming, such as battlefield 2, team fortress 2, halo 2, etc. I can still run them now, but it can get laggy sometimes with the graphics and it doesn't feel smooth playing them. My computer specs are:
AMD Athlon X 2 5050e @ 2.6 Ghz
Ati Radeon HD 3200 256mb
4GB of ram.
320 GB harddrive


for some reasom, my integraded HD 3200 graphics has 2.6 GB of shared ram. I think its the GPU problem because i'm only doing light gaming. Also, i only have about 80$ to spend on uprgading? upgrading to what would make the most out of my investment?

Thanks