I would highly recommend coming over to this
computer forum I am a part of. There are people there that can recommend parts to build a spec for the best deal and can tell you if all the parts you have selected will work together. Some people on here think of me as having a decent knowledge of computers but this forum is brilliant.
This could be considered your 'dream computer', expensive but awesome.
CPU - i7 930 - £220.85
Mobo - Asus P6TD Deluxe X58 Socket 1366 8 Channel Audio ATX Motherboard - £216.33
RAM - Corsair 6GB (3x2GB) DDR3 1600MHz XMS3 Memory Kit CL9(9-9-9-24) 1.65V - £111.64
GPU - GTX 460 1GB x2 in SLI - £348.92 OR
ATI 5850 x2 in crossfire - £423.84
PSU - OCZ GameXStream 850W PSU - Silent 120mm Blue LED Fan 4x PCI-E 6x SATA - £84.99 (some will say that's overkill but if you want to overclock or add a second or third card you will be glad of the headroom)
Case - Antec P183 Case - GunMetal Grey Super Mid Tower - No PSU - £111.65
HDD - Samsung F3 1TB - £43.79
Optical drive - LG CH08LS10 8x BD-ROM with DVD±RW & LightScribe SATA Blu-Ray Drive - OEM Black - £48.99
Sub total - £1190.72 - £1262.08 depending on GPU choice
Still needs operating system, windows 7 64bit is a must in my opinion
places to buy compoents I would recommend are
overclockers Scan and
ebuyer