Following on from a recent(ish) discussion about these.
Ebuyer now have the cheap HP Dual Core Opteron servers back in stock, now at £97.86 +VAT but including a 3 year next business day warranty package.
http://www.ebuyer.com/product/146548
Ram is a bit lacking at 512MB but it is cheap to add more
They are linux certified by HP for some flavours of Suse and Redhat so linux friendliness shouldn't be an issue and although the built in Graphics is a bit wanting they do have a full length PCI-E x16 slot to add something better so you could convert them into a reasonable desktop.
The only negative thing I have to say is that the particular chip they use is (as Adam Bower has pointed out previously) known to be a bit of a watt muncher so expect a reasonable tag on your electricity bill if you use them as a home server powered 24/7. Probably an idle of something in the region of 200Watts or maybe slightly less.