Well, seeing as I run AutoCAD off a home computer build, I can recommend highly what you'll want.
First step - Multiple Monitors, You'll want to use an Nvidia CUDA supported card so you can run 3 monitors. 2 monitors is bare minimum for CAD because you can transfer different drawing segments to different screens. VERY important for user-productivity. As far as a graphics card, I would recommend at least one of the 5xx series, preferably a 560, or 570 series. The reason for this, to get more then two monitors with an AMD card, you have to buy an active link Mini-Display-Port adapter which runs right around $100-130 for the bare minimum, whereas with an Nvidia display setup, you can just hook up 3 monitors and go. Being an AMD fanboy doesnt cut it for an actual workstation, you wont be running designs any quicker with a pricier card. Don't worry abot getting an FX series card, those are absolute work beasts for doing video design and editing/photoshopping, not needed for CAD work.
Get an SSD (Solid-State Drive). Again, its all about productivity and making sure things load up and transfer quickly, a simple 128gb Sata 2 or Sata 3 SSD will do wonders for making things run smoothly.
CPU - 4/6 cores cores, all around, get yourself either an Intel or AMD chip, if your getting an AMD - I would recommend the 1060T, for Intel either an i5 2600k, or an i7.
RAM/Memory - 4 gigs works for smaller projects, but try not to skimp and get yourself 8+ gigs of memory to work with, even a medium sized project can bog you down if you aren't careful.
The most important things all around are listed there, for running Auto CAD. VERY IMPORTANT - make sure you actually enable CUDA support in the Software, You have to manually go into the program settings to enable it.. that alone will make a night and day difference!