The top CPU you can put in there is a P4 3.4Ghz, available on Ebay for between £35~45. But I would say you are probably better off saving your cash towards a new build as P4 is already two generations old, and in todays terms is a bit of a dinosaur. The same amount of money would buy you a second hand Core2Duo chip, add another ~£30 for a second hand motherboard and ~30 for a couple of gigs of DDR2 and you would have a system that would blow a 3.4Ghz P4 out of the water!