You may want to look at a workstation card for those apps.
Something like a Quadro (Nvidia) or FireGL/FirePro (ATI).
Expensive cards but they're specifically made for CAD apps and while not any gamer video card, they do what they're designed to do much better than the GeForce/Radeon equivalents. I'm no expert and have little to no experience with CAD programs and 3D modeling...I'm sure someone else can back me up on this, or explain in better detail?
The GeForce GT 120 is a fairly weak card by today's standards though. By doing a quick search, you'll find it's a rebranded GeForce 9500 GT...
The workstation cards are pretty much the same card with different bios and drivers that are made to perform better in professional applications rather than games. I believe you can find hacks to make your gaming card into a workstation card. I'm pretty sure that voids the warranty though.
As for GT 120 vs. 4870, that's easy. The 4870 will smoke that GT 120 in anything that uses a graphics card.