Like Yewen says just add CPU, hard drive, optical drive and Ram and away you go. I'd go for something like this given the tight budget, it uses onboard graphics and I'd prefer to have 1gb Ram for even a basic system now but it can't be done with the price. You might also check if it has SATA ports, OcUK says it does, the website says 2x UDMA133 under SATA which cannot be right since SATA is 150 or 300(nominal transfer rates anyway)
FS-030-AS Asus Vintage V2-AE1 Barebones System - AMD Socket 939 (FS-030-AS)
£54.95 £54.95
CP-118-AM AMD Athlon 64 3200+ Venice (Socket 939) - Retail (ADA3200BPBOX) (CP-118-AM)
£49.95 £49.95
HD-023-SA Samsung SpinPoint P HD080HJ 80GB SATA-II 8MB Cache - OEM (HD-023-SA)
£25.95 £25.95
MY-002-OK OcUK Value 512MB PC3200 184pin DDR Memory (MY-002-OK)
£28.95 £28.95
CD-020-SO Sony DDU-1615 DVD-ROM (Black) - OEM (CD-020-SO)
£10.50 £10.50
Subtotal £170.30
VAT £29.81
Total £200.11