Under 400quid?! Just had the same limits to suggest to my friend:
Assuming you have keyboard, mouse, monitor:
CPU:
http://www.overclockers.co.uk/acatalog/Socket_939_cpus.htmlAMD Athlon 64 3000+ Venice 90nm (Socket 939) - Retail
Price: ?88.95 (?104.52 Including VAT at 17.5%)
MOBO:
http://www.overclockers.co.uk/acatalog/ASUS_Socket_939.htmlAsus A8N-E nForce4 Ultra (Socket 939) PCI-Express Motherboard
Price: ?59.95 (?70.44 Including VAT at 17.5%)
HD:
http://www.overclockers.co.uk/acatalog/Maxtor2.htmlMaxtor DiamondMax Plus 10 NCQ 80GB 6L080M0 SATA 8MB Cache - OEM
Price: ?34.75 (?40.83 Including VAT at 17.5%)
Cables:
http://www.overclockers.co.uk/acatalog/sata_cables.htmlAkasa Serial ATA 45cm Silver Data Cable
Price: ?3.50 (?4.11 Including VAT at 17.5%)
RAM:
http://www.overclockers.co.uk/acatalog/Crucial_Value.htmlCrucial 512MB DDR PC3200 CAS3
Price: ?29.95 (?35.19 Including VAT at 17.5%)
Get two 512mb's its cheaper than one 1gb!
PSU:
http://www.overclockers.co.uk/acatalog/Akasa_PSU.htmlAkasa Ultra Quiet 400W Paxpower Active PFC ATX2.0 PSU - Black Nickel with Blue LED Fan
Price: ?39.95 (?46.94 Including VAT at 17.5%)
Graphics:
http://www.overclockers.co.uk/acatalog/6600.htmlLeadtek WinFast PCX6600TD GeForce PCX6600 256MB DDR TV-Out/DVI (PCI-Express)
Price: ?69.95 (?82.19 Including VAT at 17.5%)
TOTAL: ?384.22
:D:D:D Best you can get budget wise, might be able to find the parts cheaper elsewhere?!