Here is what I'm using. It still works great, and the parts are cheaper now. Unless you're scared of putting a computer together (it's pretty easy compared to designing a robot), you should order parts online and assemble yourself.
http://www.chiefdelphi.com/forums/sh...09&postcount=9
The most important part of the computer is the video card. Get a Quadro or FirePro card in the appropriate price range. They are both good. Spending more on the processor than you spend on the video card is a bit of a waste if you're only using it for CAD. I'm using an AMD Phenom II (several years old, $90), and it works great. It runs the most complex models I throw at it without annoying me, and I am easily annoyed by slow computers.
Here's a guess at what I would be planning to spend if I was buying a CAD computer again. Given how well this one is working for me, I wouldn't do anything differently.
Video card: $160
CPU: $100
Motherboard: $50-100
Hard drive: $50
8 GB of Memory: $40
Power Supply: $40
Keyboard: $8
DVD drive: $20
Mouse: $8
Tower/Case: $60