Just going as a base unit specification I'd aim for something like this as a good balance between performance and price, there are of course faster options in some areas but price can get a bit out of hand.
Lian-Li PC-7 PLUS Aluminium Midi-Tower Case - Black  £48.99
(£57.56)  £48.99
(£57.56) 
 Corsair HX 520W ATX2.2 Modular SLI Compliant PSU (CMPSU-520HXUK)  £61.99
(£72.84)  £61.99
(£72.84) 
 BFG GeForce 8800 GTS 640MB GDDR3 HDTV/Dual DVI (PCI-Express) - Retail  £294.99
(£346.61)  £294.99
(£346.61) 
 Corsair 2GB DDR2 XMS2 Dominator PC2-6400C4 TwinX (2x1GB)  £174.99
(£205.61)  £174.99
(£205.61) 
 Intel Core 2 DUO E6600 "LGA775 Conroe" 2.40GHz (1066FSB) - OEM  £177.99
(£209.14)  £177.99
(£209.14) 
 Asus P5W DH Deluxe WiFi (Socket 775) PCI-Express DDR2 Motherboard  £129.99
(£152.74)  £129.99
(£152.74) 
 NEC AD7170A 18x18 DVD±RW Dual Layer ReWriter (Black) - OEM  £18.50
(£21.74)  £18.50
(£21.74) 
 Seagate Barracuda 7200.10 320GB ST3320620AS SATA-II 16MB Cache - OEM  £59.99
(£70.49)  £59.99
(£70.49) 
 Tuniq Tower 120 CPU Cooler  £28.99
(£34.06)  £28.99
(£34.06) 
     Sub Total :  £996.42 
Shipping cost assumes delivery to UK Mainland with:
City Link Parcel Next Day (Delivered Mon-Fri)
(This can be changed during checkout) Shipping :  £10.95 
  Vat :  £176.29 
  Total :  £1,183.66