In the interim, have you considered throwing some more RAM at the old one? My main PC is an Athlon 2800XP with a 256mb Radeon 9600 and after biting the bullet and installing 1GB of RAM it flies along even now, two years after creation and would play pretty much anything you care to throw at it, with very good frame rates.
If you don't like the idea, let me be the first to offer you 50 quid for the box.
Conan