Running ESXi 4.0 on an HP DL380 G6 at home. The server has one 2.26GHz E5520, 4GB UDIMMs, two 146G 10k 3G SAS disks. Almost forgot - ESXi is installed on an SDHC card in the embedded SD slot.
Next year is the year of the upgrade: Expanding to eight drives, 24G RDIMMs, second CPU, second PSU, BBWC.
It was on sale couple weeks back in that price range with those specs, that'd be my guess.
Always keep an eye on sales, always.