To be honest, I very much doubt you would notice any performance difference between the XP and 64 models unless you used a benchmark system (well certainly not £160 worth of difference). Similarly if you changed to a P4 3GHz.
When you do upgrade, I think you should go 64 bit but I would follow Bremmer's advice and hang fire for a short while yet.
Have to agree with the others here, I too don't think you will see any difference in performance. I've got a Athlon 2500 which is quite a bit below what you have already and to be honest I doubt I would see the benefit if I upgraded. Bear in mind the longer you wait the faster processor you will get for your money as when new ones come out the old ones drop in price. Personally I'd wait maybe 6 months or so and see what the prices are then, you would either be able to get the setup you describe now for a lot less or get a much faster system for the same money.