Ok #1: For gaming AMD is REALLY the way to go, your dumb / fanboy if you dont. Get an Athlon 64, not really fammilar with UK prices so try to get a venice core 3200, 3500 or a 3700+ San Diego core.
#2: Motherboard, I recomend the DFI Lanparty SLI-DR. You want a board that supports PCI-E, not PCI-X. PCI-X is completly dififerent.
#3: You should be able to afford a 7800GT / 7800GTX. Try to look for evga, leadtek (winfast) or BFG.
#4: AMD systems only support DDR and not DDR2 now, but due to the intergrated memory controller DDR400 peforms better than DDR533 on an intel system. Try to get 2x 1 gig sticks