The cheapest upgrade you can do is to buy a mobo that supports an AGP card but will take your CPU and ram. You can then buy an AGP card. The mobo will probably have to be used, the AGP card could be new. It won't be great, but it should let you play 800x600, or 1024x768 if your lucky. Depending on the board and GPU you buy this will probably run around $100-$150.
You're better off saving up more money and getting a new mobo with a CPU, ram, etc. You can reuse your drives, but you'll need about $300-400 for this option.