have a look here
http://www.misco.co.uk/
I have a Systemax 2.8GHz P4 that I bought from them about 14 months ago that has been fine. It now has an Asus video suite graphics / capture card + 1GB memory and array of internal and external hard drives totally around 600 GB ,Wireless keyboard and DVD writer plus a Firewire / multi-USB2 card.
Does everything I need though another external drive might be needed soon as the reality tv shows are well on the way to filling the 250GB external one.
Another source is Stak trading in rugby.
Have 3 PCs this one (see above) ,and old 1GHz Athlon that spends all day collecting Emails ,and a Dual 2GHz Athlon MP which doesn't get used as much as it used to but again has a Video suite and loads memory etc.