Spend the maximum amount of money you can to allow you to get the best possible tripod. Do not cheap out on a tripod since you'll only end up spending more money to replace it once you become dissatisfied. Manfrotto and Gitzo are the two top large commercial brands IMO. I've used both and currently have a Gitzo. I'm also using a third-party (Really Right Stuff) ball head and quick release system, but you can get away with some of the first-party stuff too.

For greater rigidity and stability you generally have to sacrifice a little bit on portability - wider leg tubes and less sections (3 instead of 4 or 5).

Read Thom Hogan's excellent Tripod 101 article:
http://bythom.com/support.htm