My recommendation would be to build the PC around the Phenom II X6 1055T with at least 8GB of RAM. You don't need that much GPU power since it isn't a gaming PC. so concentrate on your Processor power and RAM.
The last 3 CAD computers I've built for my company have been a little over $1200. I used i5-750 CPU, 8gb ram, Gigabyte motherboards, usually FireGL V3600 GPU's, 64 bit Win7, Antec 300 cases, Xigmatek cpu coolers and extra case fans.
AutoCAD will doesn't have any special requirements for hardware up to a point. With the 32 bit version you can get by with any normal cheap POS video card, after all lots of people run it on ordinary laptops. If you go with the 64 bit version then appropriate video card drivers are harder to find so we found that we had to stick with a video card that Autodesk had tested and certified.
We got by for a long time with 32 bit Autocad, and still do for our work, but we are doing some work in Revit now and our Revit models worked better in the 64 bit version with 8GB of ram so we built new machines accordingly.
The most important thing in Autocad is CPU horsepower. My workstation is older and has a Core 2 Duo running at 3.8GHz. Our new i5 machines can barely beat it. If you aren't doing any rendering then at this time the quad core doesn't benefit you so you could even go with the faster i3 dual core processors. Other than rendering, autocad will run just as fast on one core as it will on 6 cores.
Our latest machine we built with a Quadro FX380 GPU, which is also certified by Autodesk. It was a little less than the FireGL and so far is working fine. But like I said you buy the fancy video card because it is certified to work with autocad, not because it helps performance in any way.
One of our new machines was built with an SSD for the OS/program drive, but so far we can't tell that it is any faster than the machines with mechanical hard drives.