I have a P55 Classified 200 motherboard with 16 GB unbuffered ECC memory and a Xeon CPU in it, which, although not officially supported, works fine.
 
My question is, do the X79 motherboards actually support ECC and VT-d?  I am looking to build a workstation-like system with 3 NVIDIA Tesla cards in it, and possibly a low-end graphics card (probably a Quadro) for display purposes.  This would allow one or two of the Tesla cards to be dedicated to a virtual machine.  The best I've seen are motherboards from ASUS, Supermicro, and Tyan, but they only support 2 Tesla cards at most, unless I got a large, 2-CPU motherboard with both sockets occupied, which is overkill for me and way more expensive than 1-CPU motherboards.
 
The X79 Classified looks particularly interesting, but I really want something that has ECC and VT-d support that actually works.  Otherwise, there's no point in upgrading from my P55 Classified 200.

Just as with your P55/Xeon combo, the X79 will support ECC in UNBUFFERED flavors up to 1600 MHz. The MoBo's maximum (32 on some, 64 on others) still apply, though, despite the Xeon IMC being able to run much more.
 
The E5-1660 is a good choice. It's only a Hex, but you've 3.3 Ghz as a base frequency. If you want a real 1P WS that'll stand anything else on its head, put your money on the E5-2687W...