If I were building a GPU box on the cheap, I'd start with a used Dell Precision T7xxx series off of Ebay for a <$300 including RAM and a Xeon or two. They have 1100 watt power supplies and lots of slots and the RAM is usually ECC. Odds are it will come with DVD burner and possibly a hard disk and a Windows license...again all for a couple of hundred dollars.

Leaving $700 for GPU's while providing reliable high end hardware pretty much designed to run GPU's for the base platform.