You haven't given much of a budget to work with considering the fact the monitor takes up about 1/3 of it but this ought to be suitable. I'd prefer to include a bigger hard drive as well but there simply isn't enough room in the budget.
MO-015-AC Acer AL2016Ws 20" Widescreen LCD Monitor - Black/Silver (MO-015-AC)
£244.95 £244.95
MB-042-GI Gigabyte K8N51GMF-9 nForce 6100 Micro ATX (Socket 939) PCI-Express Motherboard (MB-042-GI)
£44.95 £44.95
CP-121-AM AMD Athlon 64 3700+ San Diego 90nm (Socket 939) - Retail (ADA3700BNBOX) (CP-121-AM)
£127.95 £127.95
MY-046-GL GeIL 2GB (2x1GB) PC3200 Value Dual Channel Kit CAS3 (GE2GB3200BDC) (MY-046-GL)
£89.95 £89.95
GX-054-PC PowerColor ATI Radeon X800 GTO 256MB GDDR3 TV-Out/DVI (PCI-Express) - Retail (GX-054-PC)
£64.95 £64.95
CD-026-NE NEC ND4551 16x16 DVD±RW Dual Layer LabelFlash ReWriter (Black) - OEM (CD-026-NE)
£25.95 £25.95
HD-023-WD Western Digital Caviar Special Edition 80GB 800JD SATA-II 8MB Cache - OEM (HD-023-WD)
£29.95 £29.95
CA-048-AN Antec SLK3800BUK Black Quiet Midi Tower Case - 400W SmartPower PSU (CA-048-AN)
£51.95 £51.95
Subtotal £680.60
VAT £119.11
Total £799.71