I'd go for something along these lines if gaming is the primary objective, this assumes you have suitable Ram already and does include a PSU.
MB-129-AS Asus A8R-MVP Crossfire (Socket 939) PCI-Express Motherboard (MB-129-AS)
£69.95 £69.95
CA-005-SK FSP Sparkle AX500-A Blue Storm 500W ATX2.0 PSU (CA-005-SK)
£49.95 £49.95
CP-121-AM AMD Athlon 64 3700+ San Diego 90nm (Socket 939) - Retail (ADA3700BNBOX) (CP-121-AM)
£134.95 £134.95
GX-048-PC PowerColor ATI Radeon X1800 XT 512MB GDDR3 AVIVO TV-Out/Dual DVI (PCI-Express) - Retail (GX-048-PC)
£234.95 £234.95
Subtotal £489.80
VAT £85.72
Total £575.52
If you want dual core I'd go for something like this.
MB-129-AS Asus A8R-MVP Crossfire (Socket 939) PCI-Express Motherboard (MB-129-AS)
£69.95 £69.95
CA-005-SK FSP Sparkle AX500-A Blue Storm 500W ATX2.0 PSU (CA-005-SK)
£49.95 £49.95
CP-134-AM AMD Athlon 64 X2 Dual Core 3800+ (Socket 939) - Retail (ADA3800BVBOX) (CP-134-AM)
£184.95 £184.95
GX-042-HT HIS Excalibur ATI Radeon X1800 XL 256MB GDDR3 AVIVO TV-Out/Dual DVI (PCI-Express) - Retail (GX-042-HT)
£179.95 £179.95
Subtotal £484.80
VAT £84.84
Total £569.64