Overall, an excellent build. Everything there is just right for what your doing. Getting an ATI card would be a mistake, as while they excel at gaming, media and multiple monitors, they're pretty bad for CAD. The 460 is a great card.
6 GB is generally considered the minimum for i7 systems. The only amount below that would be 3 GB, which wouldn't make a very good system. The only thing I'd change is to pick up this Corsair XMS3 3x2 GB 1600 mhz CAS Latency 7
set of RAM. They'll run a touch faster, and after the rebate (which Corsair's been good about in my experience) they're the same price.
If you needed to lower costs and were sure you don't want to add a second 460 later, you could switch the HDD/PSU combo to this one: F3 1 TB and OCZ Fatal1ty 550W
. That would save you $20.