I can build you a system here, but I need to know what you are looking at for budget figures and how important it is that everything goes really snappy.
In particular, what CAD program and how much of the value of the PC to you will center on your good experiences with it.
The name of the residential building estimating software would be nice too.
Pretty much the entire price of the computer is going to be determined by what your CAD experience. If you weren't going to be using CAD at all, you could probably get by with a $250 - $300 DELL.
If you want to be doing really high end CAD stuff, it will be quite a lot higher.