I still use my P4 with an AGP mainboard for gaming, and can suggest the following:
1. Get more RAM. 1 GB total, at least. I'd suggest
http://www.crucial.com and either enter your motherboard manf. and get a suggestion, or download and run their memory scanner, which will tell you EXACTLY what you have and what you need. Crucial RAM is priced well, is shipped promptly, and has a lifetime warranty.
2. New Video Card: I used to have a 9800XT card, and it was OK. I now use an nVidia 7600GT card which runs AH much better. XFX sells two different versions (one clocked higher than the other) at NewEgg, along with another good brand that escapes me at the moment. Expect to spend around $120 to $150 depending on the clock speed. As a bonus, the 760GT makes less heat and uses less electricity than the ATI 9800.
-Llama