Hi there. Let's try and get your core stuff for £350, and then get you a new case and PSU.
Motherboard:
Abit IP43 £46.
CPU:
Intel Q6600 £120.
RAM:
2 x 1GB Corsair TwinX £26 (you only need more if you
know you need more).
Graphics:
Palit 4850 (with an upgraded HSF) £102.80
Hard Drive:
Western Dig 640GB AAKS Caviar Blue £48 (and can often find on 'Today Only' cheaper).
This comes to
£352, which includes about £8 postage (which we at Hexus dont have to pay!).
So let's have fun with the rest
Case:
Antec NSK4000 MiniTower £31.
PSU:
Enermax Noisetaker 485W £50. I must admit to being less than 100% certain about this PSU, but it's something other than the oh-so-obvious Corsair recommendation!
Xigmatek HDT-S1283 for the CPU £18.
Grand total = £450