There are TB motherboards, but if you're looking for 64GB or more, you need to go with either x79 2011 socket or a Xeon board. I don't believe they have any yet with TB. Having said that, I think I saw PCIe cards with TB support that could be added on.
Also, if you want 8 cores, the Xeon E2690 and E2687W (both socket 2011) are the only options I know of. After looking at various options, for photo work, I think an OC X79 board like the Asus P9x79-E WS with an i7 3930K clocked to about 4.5GHz will perform just as well for less than a 1/3rd of the price (board and chip). Also, you can use some pretty fast RAM,64GB for about 700 bucks!
I am looking to build one as well. But realised that I will wait a couple of months to a) see what the new Mac Pro looks like, and b) the new intel IVB-E chips, as well as the E5 26xx v2 (the 12 core in the new Mac Pro). Reason for waiting on the intel chips is native PCIe 3.0 support, which appears a little dicey right now.
I am also stuck on graphics card choices. OpenCL seems to be the way to go for photo work. The Radeon 7970 cards are cheap and appear great. The Nvidia cards are highly rated for games etc., but appear to favour more gaming and 3D than Photoshop, C1 etc. Major drawback on these is 10bit (same as 30bit) colour support, where you need to move up to the "workstation" cards. Only the FirePro and Quadro support 10bit colour and are quite a bit more expensive for very similar hardware. In itself this is not really an issue, but high-quality drivers and application support continues to drag behind. We are also supposed to have the next gen cards out in september, so again, waiting a couple of months makes sense to me.
On Storage, PCIe 3.0 RAID cards are also just beginning to show up, and if you're using SSDs in a stripe, you will hit the PCIe2.0 ceiling. TB bandwidth is even less. Again, for me, native PCIe 3.0 is kind of important. This also affects most of the top end gaming and workstation graphics cards.
Prices, I suspect will be about the same for the new stuff as the current stuff.
From what I spec'd out an i7 3930K system w/64gb and a 7970 card w/5GB will run about 2.5K and would be as good as any current macpro but with lots of expansion, compatibility, and upgrade options available. Storage - you can spend as much as you like.
A top end Dual XEON (total of 32 cores) w/128GB and FirePro W8000 would be run about 6.5-7K, but this is in a different league. Again, if I was spending this kind of cash, I'd wait for the 12 core v2 of this CPU, the E5 2697).
Boards are also showing up with the new LSI 2308 RAID controller, so you have the option of getting a PCIe 3.0 Raid controller built in for almost no additional cost if you need a robust RAID solution.
My plan would be to use Windows 7 Professional 64bit on either system, no Hackintosh for me.
So, I figure, I'll give the new Mac Pro a look first, just in case it comes in super cheap or can make tea on the side.....