A really low-budget system could go as follows:
Corsair 2GB DDR2 Value Select PC4200 Dual Channel Kit (2x1GB) (VS2GBKIT533D2) £45.99
(£54.04)
Maxtor Diamondmax 20 160GB STM3160211AS SATA-II 2MB Cache - OEM £26.99
(£31.71)
Samsung SH-S203BEBN 20x20 DVD±RW Dual Layer Serial ATA ReWriter (Black) - OEM £15.99
(£18.79)
Antec NSK1300 ATX Cube Case - 300W PSU £49.99
(£58.74)
Abit IP-95 Micro ATX (Socket 775) PCI-Express DDR2 Motherboard £31.99
(£37.59)
Intel Core 2 Duo E2140 "LGA775 Conroe" 1.60GHz (800FSB) - Retail £45.99
(£54.04)
Sub Total : £216.94
Shipping cost assumes delivery to UK Mainland with:
City Link Parcel Next Day (Delivered Mon-Fri)
(This can be changed during checkout) Shipping : £9.95
VAT is being charged at 17.5% VAT : £39.71
Total : £266.60
Leaving you to decide how much is to be spent on what size monitor
Edit: You could also go for a really budget graphics card, such as:
Asus GeForce EN7100GS 512MB Turbo Cache DDR2 TV-Out/DVI (PCI-Express) - Retail £24.99
(£29.36)
Asus ATI Radeon X1050/TD 256MB HyperMemory TV-Out/DVI (PCI-Express) - Retail £28.99
(£34.06)
Note the 7100GS is currently out of stock