Well, some good news and some bad news.
Good news: Your system does appear to have an empty AGP slot for upgrading your graphics. Are you currently using the onboard, or have you already upgraded once?
The bad news is your CPU and your ram (I assume you already know this). They are not really game-friendly.
An upgrade to 1Gb of ram and a new graphics card would greatly improve your performance, but in some games you are still going to be CPU limited. But I personally would not upgrade your CPU though. I believe it is a socket 754 board, so if you upgrade your CPU you are spending money on a processor that is two generations behind.
If you are tight for cash (i.e., don't want to buy a new computer yet), and don't play too many graphic intensive games, you could probably extend the life of your system by going to a gig of ram and getting a stronger AGP card. What we need is a budget.
Just had a quick look at newegg, here is a nice deal :
http://www.newegg.com/Product/Product.asp?Item=N82E16814161172R
But be forewarned, your Power supply may not be able to handle the stess of a more powerful graphics card. Open up your case and have a look at the sticker on the PSU. How many watts is it rated at?