Not the ultimate rig but here is some of what I bought recently.
Motherboard:
EVGA Classified SR-2 (Super Record 2) ($599)
CPU:
Intel Xeon X5690 Westmere-EP 3.46GHz 6 x 256KB L2 Cache 12MB L3 Cache LGA 1366 ($1670)
Memory:
CORSAIR DOMINATOR GT 12GB (6 x 2GB) 240-Pin DDR3 SDRAM DDR3 1866 ($259)
or
Memory:
CORSAIR DOMINATOR 24GB (6 x 4GB) 240-Pin DDR3 SDRAM DDR3 1333 ($305)
Total: $2500
Spend the rest on a good video card.
I guess you already have monitor and a PSU you can use. Probably need to buy a new case to fit the MB though as it needs room for 10 pci slots.
Then upgrade with another CPU and memory when you need. With two Xeon processors (12/24 cores) and 48GB memory you should have hardware that last for at least the next couple of years.