That processor is a bit to old for modern games. If you want to, get a new motherboard, make sure it supports your RAM and has AGP. Then get a new processor.
AMD Athlon XP 3000 £100
Asus A7N8X-E Deluxe nForce2 £50
The processor you've got will be use an older socket and it'll nearly be impossible to find another one, and it would still probably be underpowered. If you want to go a bit further up the technology line for something that'll last longer then i'd recommend:
Gigabyte K8NXP-9 nForce4 Ultra £120
AMD Athlon 64 3500 £200
It's your choice what to do, but if you are a gamer, you can't really go much further with that processor. Another 512mb of RAM would help as well, but it's not necassary. What type of RAM do you have anyway?