(I made this list before Random asked about monitor etc)
If you don't need a monitor and are building it I'd suggest something like:
CPU: Intel Core 2 Duo E8400
Graphics Card: Nvidia Geforce 9800GTX
RAM: 4GB Crucial Ballistix Tracer Red PC6400 DDR2
HDD: 1TB Samsung Spinpoint SATA
Motherboard: Asus P5N-T Deluxe nForce 780i
DVD-RW: (Any, they are like £20)
Mouse: Logitech G5
Keyboard: Logitech G15
PSU: OCZ GameXStream 600w
Case: (Obviously up to you, but I like the NZXT Hush)
All comes, with VAT, to about £940, then download a dodgy copy of Vista Ultimate 64 Bit.
If you want it pre built you'd be looking at a lower spec as obviously they charge you extra to build it, the site B00rz got his from seemed good value, he'll chime in and post it probably, otherwise there's pcspecialist.co.uk, you can tell them an exact spec and they'll build it, or theres some good pre-built ones on overclockers.co.uk
If you need a monitor as well this looks like a good choice:
http://www.overclockers.co.uk/showprodu ... 81&subcat=I'd never buy from like Dell or other names like that because they are usually ridiculously overpriced.