Assuming you already have a monitor, a computer case you can gut, a hard drive and DVD-RW, I would go for:-
Gigabyte GA-890GPA-UD3H AMD 890GX (Socket AM3) PCI-Express DDR3 Motherboard - compatible with the new six-core processors if you want to drop one in at a later date, also you can fit a second graphics card, has USB3, etc, etc.
AMD Phenom II X2 Dual Core 555 3.20GHz Black Edition (Socket AM3) - not a quad core but most can be unlocked to work on three or four cores. People are reporting 4GHz overclocked and unlocked in the 890 chipset but you will need to additionally budget for a good after market cooler; the AMD one is crap.
Corsair XMS3 4GB (2x2GB) DDR3 PC3-12800C8 1600MHz Dual Channel Kit - I would check the compatibility but this seems to be what overclockers use in their overclocked AMD builds.
OCZ StealthXStream 700w Silent Power Supply
Asus ATI Radeon HD 5850 1024MB GDDR5 PCI-Express Graphics Card
Price including VAT and postage is £606