Decide on your spec and email it to this guy for a quote. Did me a fantastic tower.
http://www.selectcomputers.co.uk/
AMD 2400, 1Gb Ram, 2x SATA 80GB HDD in Raid0, ATI Radeon 9000, Firewire, CD, DVD. Very Happy with the results.
You could also take a look at his ebay auctions.