What about (without speakers/soundcard/keyboard/mouse);
Antec P182 Super Midi Tower Case - No PSU (Gun Metal Black)
OCZ 4GB (2 x 2GB) PC2-5400C5 Dual Channel Vista Upgrade Gold Series
Corsair HX 620W ATX2.2 Modular SLI Compliant PSU (CMPSU-620HXUK)
Western Digital Caviar SE16 500GB SATA-II 16MB Cache - OEM
Tuniq Tower 120-LFB CPU Cooler (Socket 478/754/939/940/AM2/LGA775)
Intel Core 2 Quad Pro Q6600 "Energy Efficient SLACR 95W Edition" OEM
EVGA GeForce 8800 GT 512MB GDDR3 HDTV/Dual DVI (PCI-Express) - Retail (512-P2-E801-AR)
x2
Gigabyte GA-P35C-DS3R (Socket 775) PCI-Express DDR2 & DDR3 Motherboard
£952.76
To make that cheaper, you can get the one GTX or just the GT.
Thats a very powerful spec

.
If you want it with speakers/sound card too;
OCZ 4GB (2 x 2GB) PC2-5400C5 Dual Channel Vista Upgrade Gold Series DDR2 (OCZ2VU6674GK)
Corsair HX 520W ATX2.2 Modular SLI Compliant PSU (CMPSU-520HXUK)
Acoustic Energy Aego M 2.1 Speakers (Black) - Retail
Creative Sound Blaster X-Fi XtremeGamer Fatal1ty Professional Series 7.1 Sound Card - Retail
Pioneer DVR-112DBK 18x18 DVD±RW IDE Dual Layer ReWriter - (Black) OEM
Western Digital Caviar SE16 500GB SATA-II 16MB Cache - OEM (WD5000AAKS)
Tuniq Tower 120-LFB CPU Cooler (Socket 478/754/939/940/AM2/LGA775)
Intel Core 2 Quad Pro Q6600 "Energy Efficient SLACR 95W Edition" 2.40GHz (1066FSB) - OEM
OcUK GeForce 8800 GTX 768MB GDDR3 HDTV/Dual DVI (PCI-Express) - OEM
Gigabyte GA-P35C-DS3R (Socket 775) PCI-Express DDR2 & DDR3 Motherboard
Lian-Li PC-7 PLUS II Aluminium Midi-Tower Case - Black
£1,064.36
To make it cheaper again, you can change the GTX.
With keyboard/mouse:
OCZ 4GB (2 x 2GB) PC2-5400C5 Dual Channel Vista Upgrade Gold Series DDR2 (OCZ2VU6674GK)
Corsair HX 520W ATX2.2 Modular SLI Compliant PSU (CMPSU-520HXUK)
Acoustic Energy Aego M 2.1 Speakers (Black) - Retail
Creative Sound Blaster X-Fi XtremeGamer Fatal1ty Professional Series 7.1 Sound Card - Retail
Pioneer DVR-112DBK 18x18 DVD±RW IDE Dual Layer ReWriter - (Black) OEM
Western Digital Caviar SE16 500GB SATA-II 16MB Cache - OEM (WD5000AAKS)
Tuniq Tower 120-LFB CPU Cooler (Socket 478/754/939/940/AM2/LGA775)
Intel Core 2 Quad Pro Q6600 "Energy Efficient SLACR 95W Edition" 2.40GHz (1066FSB) - OEM
BFG GeForce 8800 GT OC2 512MB GDDR3 HDTV/Dual DVI (PCI-Express) - Retail
Gigabyte GA-P35C-DS3R (Socket 775) PCI-Express DDR2 & DDR3 Motherboard
Lian-Li PC-7 PLUS II Aluminium Midi-Tower Case - Black
Razer Diamondback 3G Green 1800dpi Gaming Mouse - Retail
Microsoft Comfort Curve 2000 Keyboard (B2L-00005)
£1,022.04
Can change the keyboard to a G11, if you can budget for it.