The way I see it you have a Slot A motherboard. The fastest Slot A Athlon runs at 1000 MHz, but they are very hard to find today, and even if you find one it won't run the latest games. You also don't have enought memory for today's games, and your video by all means is obsolete. If I were you, I'd go with a Socket 754 board and Sempron - cheap, yet fast, and great for games. Or if you have the money get a Socket 939 board and an Athlon 64. And a PCI-Express videocard also. Get a new hard drive cuz this one won't be enough. In other words you need a new computer but I think it's worth it.
GB 7VT600L|VIA KT600|AMD Athlon XP 2000+|2x256MB Crucial DDR266 2.5-3-3-6 1T|80 GB Seagate ATA100|ATI 9800 Pro 128MB 256-bit|LG DVD-ROM 16x|Lite-On DVD-RW 16x DL|Samsung 755DFX 17"|Creative Live 5.1|Creative Inspire P5800|Fortron 350W|custom modded case|Windows XP