That is a seriously old computer you have there! If you had the money - buy a new system!!! It's not worth upgrading something this old, but if you really need to I would make sure that you meet the requirements for the CPU, RAM and other parts. I'm aware that for AGP, there is the 6600, 6800, 7600 from nvidia and the 3670, 4350, 4670 from ATi. Even so, it likely won't be worthwhile or even enjoyable at that quality so I really suggest you buy a new system.
You should buy a new computer. Parts that old aren't very upgradable and almost no modern hardware can work in that machine. Heck, there might be no modern hardware at all. Like _zxzxzx_ said, you might get up to some Radeon 4000 cards or so if you want that, but even then, those aren't close to being modern. They're also very old, just not as old as your stuff.
If you were to buy a new computer, could you give us a budget? We can help with it if we know a budget to work with and recommend parts for. I'd recommend starting a new thread and giving us a link to it here if you do this.