In direct reponse to the original question maybe the closest upgrade you can get for around £100 is to buy a 2nd hand P4 3.73GHZ Extreme Edition for about £130. It might go to 4GHZ with a decent cooler :>
Whether its worthwhile or not depends on what you do with your PC :>
Those run on an 1066mhz bus don't they? My MB uses a 533...