I have been using a very nice Mobile Workstation from Dell at the 'Job' for about a year now. It is very fast with AutoCAD and is a good all around machine, albeit very expensive. As of today It has white paint from a tank farm on it, a white line in the screen from ??, a small black crack in the screen on the other corner, black sand residue from an aluminum foundry in places that won't come out, dents on the side from were it vibrated off of a bucket being used as a table and 2 of the USB ports don't work. It also weighs something like a million pounds.
Moral of the story. Get a good desktop for working in the office and a cheap lightweight laptop to take with you for site work, that way when it gets destroyed you won't cry as hard.
For my startup business I ordered parts for a pretty sweet workstation grade computer I am going to build myself (following Autodesks recommended parts list where applicable), and I ordered a bottom of the line Lenovo business class laptop (Thinkpad)...still costs less than my Mobile Workstation.