Just saw this thread and I thought I would chime in...
I would suggest picking up used commercial servers online (ebay / craigslist) for cheap... for example I have 4 Dell Poweredge 2950s and I got each for around $300 shipped, you do have to do some research depending on what features you want (has to have VT-x / VT-d if you want to have advanced VM features). Going with a commercial server product can have some nice perks... for example the Dell servers have a hardware remote access card that uses a separate NIC to provide admin capabilities (including remote power on / off). They are much more resilient as well, many feature dual bios and dual redundant PSU for example. Many are already well suited to running eXSI with quad-core
There are a few drawbacks, one is the formfactor (I have a 42u server rack also from craigslist), power consumption can be high especially with quad-core xeon processors, lastly memory... most commercial servers use the more expensive ECC memory (The 2950 also requires it to be buffered)... so while you'll find cheap servers, they typically have less memory.
I will also agree that FreeNas is an amazing product... and if you want to run your own firewall / dns / proxy / etc... I would suggest looking at pfsense.