Do you really need ECC REG memory? If you need it that will greatly limit your choice of motherboards. Plain ECC should be good enough I think.
The workstations we're using at university are built with the Xeon equivalent of the i7 920, 6GB of RAM, and a Quadro VGA. We're using CATIA by the way. I've also installed CATIA on my laptop ( 2.4GHz Core2Duo and 4GB RAM ), and it's running pretty decent as long as the projects aren't getting too large/complicated.
Any rig with a 2600k/3930k should be able to handle the above-mentioned programms more than well, 16GB RAM should be plenty too. However check out which mainboard you can use for this build first!