I'd say the absolute best motherboard is the Tyan S4882UG2NR, it has:
4 x 940 pin sockets (yes that means you can put in 4 CPU's)
dual channel SCSI
16 DIMM's (4 for each CPU)
2x Gigabit ehternet ports
its downfall is that it doesn't have PCI-E or AGP (it's a server motherboard)
its price: $1409 US (cheapest I've seen)
and you can put in 4 Opteron 850's which are 8-way, meaning that each CPU looks like 8 CPU's to the PC so it processes 8 things at a time; now 4 of these means that there is 32 virtual processors in your PC!!!
each CPU costs $1469 US each (cheapest I've seen)
so the cost for the motherboard and 4 CPU's would be $7285
now I'll just be realistic, that's a ton of money for just your motherboard and CPU (or should I say CPU's), which doesn't even have AGP or PCI-E, so I would just stick with the FX-55, which you can get AGP and PCI-E boards for, along with the 2 6800 Ultra's in SLI
although I would get Kingmax PC4000 Hardcore instead of Corsair
the Kingmax PC4000 overclocks way better due to it running on lower voltage therefore running cooler, and it is also less expensive
I would get an Asetek Vapochill stand-alone cooler for your FX-55, which cools to about -15 to -30 degrees (yes that's negative) which will give you a lot of overclocking potential there
and if you want, watercooling for your 2 6800's
you can make a watercooling kit for around $250-$300 aus (or about $175-$200 US)
which would include a reservoir, pump, and the 2 GPU waterblocks (not 1 CPU and 1 GPU, since the CPU would be cooled by the Vapochill cooler (unless you don't want to get it))