Another idea, for £2k I would say go to Opty 175:
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
FG-000-AR Arctic Cooling Freezer 64 Pro CPU Cooler (Socket 754/939/940) (FG-000-AR)
£14.50 £14.50
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-046-WD Western Digital Caviar SE16 250GB 2500KS SATA-II 16MB Cache - OEM (HD-046-WD)
£60.95 £60.95
HD-007-WD Western Digital Raptor 36.7GB WD360GD 10,000RPM SATA 8MB Cache - OEM (HD-007-WD)
£63.95 £63.95
CD-026-NE NEC ND4551 16x16 DVD±RW Dual Layer LabelFlash ReWriter (Black) - OEM (CD-026-NE)
£27.95 £27.95
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
MO-009-NE NEC MultiSync® LCD20WGX2 20.1" Widescreen LCD Monitor - Black/Silver (MO-009-NE)
£349.95 £349.95
MY-002-MK Mushkin 2GB DDR XP4000 Redline Extreme Performance Dual Channel Kit (2x1GB) (991493) (MY-002-MK)
£214.95 £214.95
Subtotal £1,604.00
VAT £280.70
Total £1,884.70
Still leaves spare cash for new keyboard, mouse, speakers etc.