I'd grab a Debian testing CD image, with LXDE/XFCE desktop environment (try LXDE first).

If that doesn't fits, then you need to try minimalistic distros (I'd also try with Arch Linux but I think they dropped support for older proccessors).

Wow, thanks very much. Now I have a lot more choices to choose from. I'm also thinking of upgrading my HDD, I found 30GB HDD for 12 pounds shipping included, I think its a pretty fair price. :)