If you want to go pretty cheap, but you are still looking for a decent upgrade from your stock on-board graphics card, you could buy a GeForce Graphics Card. I bought one (can't remember the model number and everything) and it was $50, and it came with 128MB memory, even though most of the newer more advanced graphics cards come with 256MB of memory.

If you can pick up a GeForce 7600 GT or GS then that'll be the best bang for the buck. That card has 12 pipelines and is based on a fairly new architecture so you're gonna get all the shader support. My cousin has this card and it's way better than the ATI cards at the $100-$150 range.