I spent an evening pondering on pretty much the same question as Philipe. I then discovered that, going by Passmarks' CPU Mark results, a singular Core i7-3930K is faster than a dual Xeon E5645 -- and quite a bit cheaper, too.
For the motherboard I picked the Asus Rampage IV Extreme because it is currently the only LGA 2011 mobo that has enough PCI-E lanes for my needs. Barely; you can have one x16 slot and three x8 ones. That should be good enough for a Quadro 4000 in the x16 slot, followed by a RED Rocket, a GeForce GTX460 and a SCSI adapter in the x8 slots. I bought 32GB of Corsair Low Profile Vengeances because I had reason to believe DIMMs with tall heat spreaders would not fit under the Noctua cooler.