Something like this would probably do, it isn't great in any one area but it is solid enough overall and the 20" monitor is just because it is the same price as the 19" ones currently in stock.
Hitachi Deskstar 7K160 160GB SATA-II 8MB Cache - OEM (0Y30006) £26.99
(£31.71) £26.99
(£31.71)
AMD Athlon 64 X2 Dual Core 4200+ 2.20GHz (Socket AM2) - Retail £40.99
(£48.16) £40.99
(£48.16)
Crucial 2GB (2x1GB) DDR2 PC2-5300C5 Dual Channel Kit (CT2KIT12864AA667) £24.99
(£29.36) £24.99
(£29.36)
Antec NSK 4480 Mini Tower Case (Silver) - 380W Earth Watts PSU £45.99
(£54.04) £45.99
(£54.04)
OcUK Value L-2041W 20" Widescreen LCD Monitor - Black £99.99
(£117.49) £99.99
(£117.49)
Samsung SH-S202JBEBN 20x DVD±RW IDE Dual Layer ReWriter (Black) - OEM £13.99
(£16.44) £13.99
(£16.44)
OcUK GeForce 7600 GT 256MB GDDR3 TV-Out/DVI (PCI-Express) - Retail £41.99
(£49.34) £41.99
(£49.34)
Abit AN52 nForce 520 (Socket AM2) PCI-Express DDR2 Motherboard £36.99
(£43.46) £36.99
(£43.46)
Sub Total : £331.92
Shipping cost assumes delivery to UK Mainland with:
City Link Parcel Next Day (Delivered Mon-Fri)
(This can be changed during checkout) Shipping : £12.95
VAT is being charged at 17.5% VAT : £60.35
Total : £405.22