I've gone back and taken a look at the options, and the 3700+ has a cache memory of 1mb whereas the 3800 has 512. So i took your advice and went for that. So this is my updated list. Any comments?
Asus A8N-VM CSM, nForce430+GeForce 6150, Socket-939, m-ATX,GbLAN, Firew, PCI-Ex16
AMD Athlon 64 3700+ 2.2GHz Socket 939, 1MB, BOXED w/fan
Corsair TWINX2048-3200C2 DDR-DIMM 2048MB Kit w/two matched CMX1024-3200C2 DIMMs
Sapphire Radeon X1800XT 256MB GDDR3, PCI-Express,ViVo,Dual-DVI-I,Lite-Retail
that comes to just under £480