Assuming that 1 GBP is roughly equal to 2 USD...
Core 2 Duo E7200 Wolfdale dual-core - $140
Gigabyte EP35-DS3L motherboard - $90
2x1 GB kit Crucial DDR2-800 memory - $55 ($20 MIR)
XFX GeForce 8800 GT 512mb - Zalman HSF - $170
Cooler Master Centurion 534 case - $45
Corsair 550VX power supply - $70
Seagate 7200.10 250 GB hard drive - $60
You can choose your optical drives. I personally love my SAMSUNG DVD-RW.
Should round out to just under 700 USD after shipping. This is priced on Newegg.com for everything but the PSU, which was priced on Buy.com.