Hello
I'm building a system for a friend who will be using it for:
I can't speak to the other softwares, but as far as AutoCAD goes, we've been fine with a desktop build with workstation graphics. And yes, AutoCAD can only use one core for most things, although rendering and regen can use multiple cores (http://usa.autodesk.com/adsk/servlet/ps/dl/item?siteID=123112&id=15224826&linkID=9240617). If it were me, I'd go with the highest clock i7 vs. paying the extra for the server board and CPU. Again, I have no experience with any of the other programs you mentioned, though; they may have different requirements and capabilities.
My full specs are below; we mostly do 2D stuff, but 3D every once in a while.
Friend don't let friend buy Xeons. Extra cost and zip benefit for doing CAD. thow in ECC RAM, and you're jacking the cost up with squat for benefit.
I've inherited a Dell 690 workstation, dual Xeon and 6Gb RAM. Adding another 4Gb of RAM will run $300 (from Crucial, not Dell). That's not cost effective.....
Workstations are not servers. (Dell, HP, Lenovo, etc push Xeon for 'workstations' because there's a better margin of profit in it for them - not because it helps the end user get things done.