As a guy who does tech support for money and has built and upgraded a good few PCs, my advice would be to
not build a PC unless you want to do it for the sake of doing it. Unless you pirate software, the price advantage is rather slim.
Personally, I would suggest getting whatever PC meets your price requirements from HP/Compaq. Dell, IBM, and Sony really are overpriced for a home user. The default set of components are usually good, except that a lot of PCs still have a default of 256MB memory which is definitley not enough. 512 is the minimum, and at $750 (assuming you aren't getting a monitor), I'd go for 768 or 1GB. Finally, I'm not sure if AutoCad uses multiple processors very well, but if you really want this machine for CADding, a Hyper-Threaded Pentium probably is your best bet.
And BTW, if you are looking for PC components, check out this site, too:
www.pricewatch.com. If only that site existed for all products...