Ok, when I buy computers, I spend hours planning them out. What you would want to do in theory (Depends how good you are with computers) is to buy parts and assemble (what your doin I think...). Well, AMD athlon 64 is basically the best processor you can get, but the problem is, normal versions of windows DO NOT support 64 bit processing. This makes for spending an extra $200 (not sure in £). If you do not wish to pay that for 64 bit processing, then the best intel processor is the new core duo. This provides fairly fast duel core processing (Once again, duel core does NOT double application speed, it mearly speeds up when running multiple operations which is almost all the time). A cheaper way to go would be the Intel Pentium 4 Hyper Threading 3.4 Ghz (not sure exact model, costs about 140-180), or the 1.66 Ghz duel core in same price range. Both of these cause major overheating problems which means you are going to have to get a better heatsink/processor fan as opposed to the cheap packaged ones.
As for motherboard, you are looking for high bus speed with the right slot. I do not know all the slots off hand, but I believe the HT 3.4 intel pentium 4 is a slot LGA775. You should not have to spend more than about $80 on a motherboard.
Ram you should get 1 gig of DDR ram (or 512 Is ok I guess).
If you want fast graphics, You want a PCI express graphics card (DO NOT GET A REGULAR ONE

).
blah balh blah im tired of typing... so hope thats remotely helpful ><