To be honest it doesn't take much to run CAD/CAM comfortably. I personally am using an nVidia GTX 970 on my desktop at home to run Inventor, but I have also run it on a GTX 760 and 480 in the past with no significant issues. I also run Inventor on my laptop which has a Radeon HD 8650G + 8670M Dual graphics setup on it and, while not quite as smooth as my desktop, runs just fine with only minimal performance loss.
By in large, while graphics are important to CAD, it seems that most programs are more CPU and RAM dependent then they are graphics. That said, I do always recommend people watch these two videos when shopping for a graphics card for any purpose:
https://www.youtube.com/watch?v=sph6cjJeRdI
https://www.youtube.com/watch?v=xp6ltBCMDCE