AM2:
AMD Athlon 64 X2 Dual Core 3800+ 2.00GHz (Socket AM2) - Retail (CP-166-AM)
£213.79
MSI K9N Platinum nForce 570 (Socket AM2) PCI-Express DDR2 Motherboard (MB-070-MS)
£91.59
G.Skill 2GB DDR2 LA PC2-5300 (2x1GB) CAS4 Dual Channel Kit (F2-5400PHU2-2GBLA) (MY-012-GS)
£117.44
Connect3D ATI Radeon X1800 XT 256MB GDDR3 AVIVO TV-Out/Dual DVI (PCI-Express) - Retail (GX-044-CO)
£164.44
Hitachi Deskstar T7K250 NCQ 250GB SATA-II 8MB Cache - OEM (0A31636) (HD-002-HI)
£55.81
NEC ND4570 16x16 DVD±RW Dual Layer ReWriter (Black) - OEM (CD-030-NE)
£25.79
Antec SLK3800BUK Black Quiet Midi Tower Case - 400W SmartPower PSU (CA-048-AN)
£53.23
Acer AL1916WS 19" Widescreen LCD Monitor - Black/Silver (MO-012-AC)
£176.19
Total inc VAT: £898.28
939:
AMD Athlon 64 X2 Dual Core 3800+ (Socket 939) - OEM (CP-152-AM)
£182.07
Asus A8N-E nForce4 Ultra (Socket 939) PCI-Express Motherboard (MB-088-AS)
£64.57
GeIL 2GB (2x1GB) PC3200 Value Dual Channel Kit CAS3 (GE2GB3200BDC) (MY-046-GL)
£129.19
Sapphire ATI Radeon X1800 XT 512MB GDDR3 AVIVO TV-Out/Dual DVI (PCI-Express) - Retail (GX-103-SP)
£205.57
Hitachi Deskstar T7K250 NCQ 250GB SATA-II 8MB Cache - OEM (0A31636) (HD-002-HI)
£55.81
NEC ND4570 16x16 DVD±RW Dual Layer ReWriter (Black) - OEM (CD-030-NE)
£25.79
Arctic Cooling Freezer 64 Pro CPU Cooler (Socket 754/939/940) (FG-000-AR)
£17.57
Antec SLK3800BUK Black Quiet Midi Tower Case - 400W SmartPower PSU (CA-048-AN)
£53.23
Acer AL1916WS 19" Widescreen LCD Monitor - Black/Silver (MO-012-AC)
£176.19
Total inc VAT: £909.99
If it wasn't for the monitor, you could get a real nice rig with the X1900XT and dual core. Increase the budget if you can.