If you want the fastest PC at the moment then this is the quad SLI system with the PhysX card
It isn't worth £4k+ though. You can get a PC that's pretty much just as fast, with SLI and dual core, for half that
If you want the fastest around then you end up paying for it
How about:
CP-146-AM AMD Athlon 64 X2 Dual Core FX60 (Socket 939) - Retail (ADAFX60DAA6CDBOX) (CP-146-AM)
£599.95 £599.95
BU-000-PC ATI X1900 Master Card & ATI X1900 XTX Crossfire Bundle ***DISCOUNT*** (BU-000-PC)
GX-059-PC PowerColor ATI Radeon X1900 XT-X 512MB GDDR3 AVIVO TV-Out/Dual DVI (PCI-Express) - Retail (GX-059-PC) 1
GX-045-CO ATI Radeon X1900 ***Crossfire Edition*** 512MB GDDR3 TV-Out/Dual DVI (PCI-Express) - Retail (GX-045-CO) 1
£594.90 £594.90
MB-131-AS Asus A8R32-MVP Deluxe Crossfire (Socket 939) PCI-Express Motherboard (MB-131-AS)
£109.95 £109.95
MY-057-OC OCZ 2GB (2 x 1GB) PC3200 Dual Channel Platinum Series EL-DDR CAS2 (OCZ4002048ELDCPE-K) (MY-057-OC)
£149.95 £149.95
HD-069-WD Western Digital Raptor 150GB WD1500ADFD 10,000RPM SATA 16MB Cache - OEM (HD-069-WD)
£173.95 £173.95
CD-033-NE NEC ND4571 16x16 DVD±RW Dual Layer LabelFlash ReWriter (Black) - OEM (CD-033-NE)
£24.95 £24.95
CA-054-LL Lian-Li PC-7 PLUS Black Aluminium Midi-Tower Case (No PSU) (CA-054-LL)
£48.95 £48.95
CA-002-SS Seasonic S12 600W Silent ATX2.0 Power Supply (CA-002-SS)
£87.95 £87.95
Subtotal £1,790.55
VAT £313.35
Total £2,103.90
Half the price of the OCUK System but with top of the range components - FX 60, Crossfired X1900XT & X1900XTX, Raptor Hard Drive, Aluminium case, very good quality PSU, etc
Or, if you prefer nVidia:
CP-146-AM AMD Athlon 64 X2 Dual Core FX60 (Socket 939) - Retail (ADAFX60DAA6CDBOX) (CP-146-AM)
£599.95 £599.95
MY-057-OC OCZ 2GB (2 x 1GB) PC3200 Dual Channel Platinum Series EL-DDR CAS2 (OCZ4002048ELDCPE-K) (MY-057-OC)
£149.95 £149.95
HD-069-WD Western Digital Raptor 150GB WD1500ADFD 10,000RPM SATA 16MB Cache - OEM (HD-069-WD)
£173.95 £173.95
CD-033-NE NEC ND4571 16x16 DVD±RW Dual Layer LabelFlash ReWriter (Black) - OEM (CD-033-NE)
£24.95 £24.95
CA-054-LL Lian-Li PC-7 PLUS Black Aluminium Midi-Tower Case (No PSU) (CA-054-LL)
£48.95 £48.95
CA-002-SS Seasonic S12 600W Silent ATX2.0 Power Supply (CA-002-SS)
£87.95 £87.95
MB-118-AS Asus A8N32-SLi Deluxe nForce4 SLi X16 (Socket 939) PCI-Express Motherboard (MB-118-AS)
£119.95 £119.95
GX-029-GW Gainward BLISS Geforce 7900 GTX 512MB GDDR3 TV-Out/Dual DVI (PCI-Express) - Retail (GX-029-GW)
X2
£309.95 £619.90
Subtotal £1,825.55
VAT £319.48
Total £2,145.03