I've been running ESX servers for 6 years at home (sadly). I've always used HP desktops. The quality is brilliant as they're built to the same standards as their servers.
I've had an EPC40, a couple of DC7700s, and now an 8200 Elite SFF which can take up-to 32GB of RAM. It'll run for the next 3+ years. You can pick them up cheap(er) on ebay with a year or so warranty left on them too.