Since the AGP and K7 cpus a helluva lot has changed architecture-wise. Good luck finding those old socket-a cpus too.
You could make a decent performing system for about £200-300 these days. Don't expect to play crysis on anything but medium detail though. You would be able to keep the same case, hard disks, optical drives and possible PSU if it's halfway decent. That would mean just having to get a new mobo, cpu and gpu.
http://www.overclockers.co.uk/showprodu ... otherboard
http://www.overclockers.co.uk/showprodu ... -%20Retail
http://www.overclockers.co.uk/showprodu ... =GX-142-AS
That little bundle comes to just £178.57 after tax. Depending on the ram you already have you might be able to use that. but a 2x 1GB bundle costs around £30.
That lot would have you playing games like Call of duty 4 on decent settings.
Obviously, this is ripoff britain prices. In the USA you'd probably get that for like $5 or something...