Something like this might be suitable, I've left out the soundcard since for most onboard is perfectly sufficient and with the speakers you won't notice any real difference.
Gigabyte GA_965P_DS3P (Socket 775) PCI-Express DDR2 Motherboard £74.99
(£88.11) £74.99
(£88.11)
SpeedLink Instinct SL-8214 2.1 Speaker System £15.10
(£17.74) £15.10
(£17.74)
Microsoft Black Value Pack 2.0 - OEM (S82-00029) £12.99
(£15.26) £12.99
(£15.26)
Microsoft Windows Vista Home Premium 64-Bit Edition DVD - OEM - 1Pk (66I-00788) £63.99
(£75.19) £63.99
(£75.19)
Intel Core 2 Duo E6600 "LGA775 Conroe" 2.40GHz (1066FSB) - Retail £125.99
(£148.04) £125.99
(£148.04)
Samsung SH-183LBEBN 18x18 DVD±RW Dual Layer Serial ATA Lightscribe ReWriter (Black) - OEM £16.99
(£19.96) £16.99
(£19.96)
Western Digital Caviar SE16 500GB 5000AAKS SATA-II 16MB Cache - OEM £63.99
(£75.19) £63.99
(£75.19)
Antec NSK6500 Super Midi Tower Case - 430W PSU £58.99
(£69.31) £58.99
(£69.31)
Sapphire ATI Radeon X1950 Pro ULTIMATE SILENT 256MB GDDR3 HDTV/Dual DVI (PCI-Express) - Retail £89.99
(£105.74) £89.99
(£105.74)
Crucial Ballistix 2GB (2x1GB) DDR2 PC2-5300C3 667MHz Dual Channel Kit (BL2KIT12864AA663) £67.99
(£79.89) £67.99
(£79.89)
OcUK Value Hanns-G HW191D 19" Widescreen LCD Monitor - Silver £94.99
(£111.61) £94.99
(£111.61)
Sub Total : £686.00
Shipping cost assumes delivery to UK Mainland with:
City Link Parcel Next Day (Delivered Mon-Fri)
(This can be changed during checkout) Shipping : £14.95
VAT is being charged at 17.5% VAT : £122.67
Total : £823.62