if you need it now, just buy now. Sandy Bridge isn't looking to be a huge step in speed, clock-for-clock, so you'd be waiting on 8-core CPUs...but that's a ton of productivity lost just for 2 more cores.
TB is too slow for high-end graphics, PCI cards will probably be available next year, and in the meantime there's still USB 3 and eSATA 6.
note that if you build your workstations, you do the support, too. do you have time to do your own support? that would also be a reason to ditch Apple and go with Dell or HP (better business support).
are you pricing the 12-core MP with Apple parts? because there's no reason to. just get the CPUs and GPU and put in everything else yourself.
TB is too slow for high-end graphics, PCI cards will probably be available next year, and in the meantime there's still USB 3 and eSATA 6.
note that if you build your workstations, you do the support, too. do you have time to do your own support? that would also be a reason to ditch Apple and go with Dell or HP (better business support).
are you pricing the 12-core MP with Apple parts? because there's no reason to. just get the CPUs and GPU and put in everything else yourself.