No, the videocards used are different.
CAD workstations will use Quadra/Fire model cards.
Gamers will use X19x0/79x0 or 8800 model cards.
The issue is precision vs. raw speed. Gaming cards can be sloppy in their rendering but must be fast. CAD cards must be precise regardless of how fast they are.
Depends on what kind of 3d modelling are you doing. If it's high performance large assembly mechanical modelling, then you need a professional CAD video card which is very pricy, has optimized drivers and it's not that fast with the games. If you do modelling in Autodesk Inventor, 3ds Max or even light to moderate Solidworks, then a gaming card might be enough.
Usually, the hardware of professional cards and gaming cards are very similar, it's the drivers that make the difference. They are more stable, because they are more throughly tested and they are optimized for CAD work. With the same underlying hardware, the CAD cards are a couple of times faster in typical OpenGL CAD applications. Yes, it's hard to believe what optimized drivers can do, but this is what tests show. Support is much better too. I used to have a Radeon X800 pro gaming card and I recently replaced it with its hardware identical CAD version, FireGL X3 and I don't regret it. The prices for old CAD cards dropped on EBay and againg, I mostly CAD versus play, or play not that graphics intensive games, like WoW. "Precision" is much less of an issue, rather "strict conformace" to OpenGL API is.
CPU, you need the fastest either way and naturally Intel it's the best choice these days. Memory, the more the better, start with 2x1GB sticks and upgrade later to 4x1GB sticks if you need to work with large CAD assemblies. Hard disks, you may want to raid them for error redundancy versus speed.
My setup is a pretty old AGP based rig: Socket 939 dual core AMD 4600+, Asus A8V deluxe, ATI FireGL X3 graphics card, 4GB ram, and 2xWD Caviar 200GB RAID 0 harddisks. I've built it quite cheap and steadily upgraded it, initially was a single core gaming rig, that got more RAM, better CPU and video card as my CAD needs grew.
Use good quality harddisk and memory so you won't have reliability issues and you'll still end up cheaper than professional workstations that solve these issues with expensive ECC/ FB-DIMM ram and raided SCSI harddisks.