Definately get a new computer, 6 years old is quite old. You might want to get a notebook, I replaced my old notebook almost a year ago and I got a dual core Centrino CPU model for $800. --This should translate to about 400 quid for you.
If you get a notebook, get something quality, like a Toshiba.
Also, whatever you get, DO NOT GET WINDOWS VISTA!!!!! If you plan on running Windows, you'd be better off using WinXP with all current service packs and security updates installed.