I designed this build for a friend yesterday. Was based on a £500 budget. For the extra £100 budget you have id recommend adding an SSD in and obviously a case (since the build i listed below doesn't include one).
www.scan.co.uk
www.ebuyer.co.uk
www.aria.co.uk
www.overclockers.co.uk
www.amazon.co.uk
www.novatech.co.uk
www.dabs.co.uk
^^ All good UK suppliers, hunt around for the cheapest parts ^^
Gigabyte GA-Z68A-D3-B3 MoBo £75
i5-2500k £160
Corsair Memory XMS3 Classic 4GB DDR3 1600 MHz £23
Coolermaster Hyper TX3 CPU Cooler £15
Corsair Builder Series 500W Power Supply £45
MSI 1GB GeForce GTX 560 Ti £177
Samsung Spinpoint F3 500GB £31 (1TB version is £43)
£526 (with 500GB HDD without case)
Could easily fit all these parts in under £500 buying from "Today Only" offers from www.scan.co.uk
Using Today Only offers (on 26/9/2011):
Gigabyte GA-Z68A-D3-B3 MoBo £68
i5-2500k £153
Corsair Memory XMS3 Classic 4GB DDR3 1600 MHz £22
Coolermaster Hyper TX3 CPU Cooler £15
Corsair Builder Series 500W Power Supply £45
Gigabyte GeForce GTX 560 Ti OC £167
Seagete 500Gb Barracuda Hard Drive £28
= £498 (without case)