To do that sort of things, people generally use downhill bikes. These bikes have around 7 - 8 inches of travel.

Examples:

http://www.specialized.com/bc/SBCBkModel.jsp?sid=08Demo
http://www.transitionbikes.com/2007/Blindside.cfm
http://santacruzbicycles.com/v10/
http://www.norco.com/bikes/mountain/...ty/team_dh.php

You need to decide what you really want to do. A downhill bike will be a pain pedalling around town, and a urban bike would likely not be able to handle downhill biking.

Get back to us when you know what you really want.