Hows about something like this:
CP-153-AM AMD Opteron UP 175 Dual Core San Diego 2.2GHz 2MB Cache (Socket 939) - Retail (CP-153-AM)
£334.95 £334.95
MB-014-DF DFI LanParty UT NF4 Ultra-D (Socket 939) PCI-Express Motherboard (MB-014-DF)
£79.95 £79.95
GX-046-CO Connect3D ATI Radeon X1900 XT-X 512MB GDDR3 AVIVO TV-Out/Dual DVI (PCI-Express) - Retail (3055) (GX-046-CO)
£324.95 £324.95
HD-070-WD Western Digital Caviar SE16 320GB 3200KS SATA-II 16MB Cache - OEM (HD-070-WD)
£79.95 £79.95
CD-026-NE NEC ND4551 16x16 DVD±RW Dual Layer LabelFlash ReWriter (Black) - OEM (CD-026-NE)
£27.95 £27.95
FG-000-AR Arctic Cooling Freezer 64 Pro CPU Cooler (Socket 754/939/940) (FG-000-AR)
£14.50 £14.50
CA-012-TT Thermaltake VA3000BNA Tsunami Dream SuperMidi Aluminium Tower - Black (CA-012-TT)
£67.95 £67.95
CA-025-EN Enermax Liberty 500W ELT500AWT ATX2.2 Modular SLI Compliant PSU (CA-025-EN)
£63.95 £63.95
MY-057-OC OCZ 2GB (2 x 1GB) PC3200 Dual Channel Platinum Series EL-DDR CAS2 (OCZ4002048ELDCPE-K) (MY-057-OC)
£134.95 £134.95
Subtotal £1,129.10
VAT £197.60
Total £1,326.70
With that ram, cpu, mobo & cooler you'll be able to seriously clock that opty, probably to around 3ghz