If you bought the components yourself and got it built you would save around £450 (compared against the pre-built system Syk3 posted). Granted that doesn't include software or peripherals, but you have allot of room to work with on those.
However, I personally would go for something like this instead:
Intel Core 2 DUO E6400 "LGA775 Allendale" 2.13GHz (1066FSB) - Retail
Gigabyte GA_945PL_S3 (Socket 775) PCI-Express DDR2 Motherboard
OCZ 2GB (2 x 1GB) PC2-6400 Dual Channel Platinum Revision 2 XTC Series DDR2 (OCZ2P800R22GK)
Gainward BLISS GeForce 8800 GTS 640MB GDDR3 HDTV/Dual DVI (PCI-Express) - Retail
Hitachi Deskstar 7K160 NCQ 160GB SATA-II 8MB Cache - OEM (0A32728)
NEC AD7170A 18x18 DVD±RW Dual Layer ReWriter (Black) - OEM
Antec P180 Advanced Super Midi Tower Case - No PSU (Black)
Seasonic S12 600W Silent ATX2.0 Power Supply
Of course all the components would be purchased from Overclockers UK.
The difference is about £800 when compared to the original pre-built system Syk3 posted.
I won't go into a sales pitch, but I have allot of experience with building PCs. I use to sell 10+ a week on eBay (all put together myself). Plus my house is 100% smoke free
Send me an email and we can have a chat if you like.