Diamond-HP :
Right I've done one up that totalled at £527.88 but that includes the Windows 8 OS.
http://uk.pcpartpicker.com/p/1lUhE
Its an AM3+ board using a Phenom 3.4 Ghz 4 core black which means you can overclock it, you can also overclock the HD 7850 GPU to a stock 7870 (I own one of these).
The Mobo is Asus which is imo the best board manufacturer and it contains the Flash back feature which means you can update the bios to run the CPU without needing an older CPU (Which is a big problem without this feature).
I have already built a PC but I am wondering on the Graphics card as this is one of the main parts of a PC. AT the moment I have this
CPU AMD FX-6300 3.5GHz 6-Core £87.59
Motherboard Asus M5A97 LE R2.0 ATX AM3+ £58.54
Memory A-Data XPG Gaming Series 8GB (2 x 4GB) DDR3-1600 £47.90
Storage Seagate Barracuda 1TB 3.5" 7200RPM £46.80
Video Card Asus GeForce GTX 650 1GB £97.08
Power Supply XFX 550W ATX12V / EPS12V £49.86
Operating System Microsoft Windows 7 Home Premium SP1 (OEM) (64-bit) £66.78
Totals to about £455