As others have said, chances are you'll have to upgrade more than the processor, especially if that's one of the older Celerons.
If you have an AGP gfx card, the cheapest route would be to go for an AMD set up (as you can still get AGP motherboards for the AMD cpu's)
This AGP motherboard for £43 with an X2 4200 for £45
You'll also need a cpu cooler for the above but they can be had for as little as £5. The only problem is these products are socket 939 which are end of the line. Should you want to upgrade again in the future, you'll need another new mobo as 939 cpu's are no more. Will still be MUCH quicker than your present set up and should be ok for a couple of years.
For a little more you can go the AM2 route, which is more future-proof but will require new DDR2 memory.
AsRock AGP AM2 mobo for £36 AMD X2 4200 AM2 £50 Corsair 1GB DDR2 £33
This will take you a few quid over budget with P&P, but will allow you to throw in bigger processors in the future. There are still some very good AGP cards around too, so that shouldn't be a worry (not sure we'll see any DX10 AGP cards released though)