This will tell you everything you need to know:
http://www.wheelpro.co.uk/wheelbuilding/book.php
I went for Mavic A319 rims (36 spoke, and you are limited to >28mm wide rubber) and DT Swiss competition spokes. They are the second pair of wheels I have build and 20st of me hitting potholes and dropping off kerbs has not knocked them out of true yet.