Choosing the right size frame
Common wisdom is that upper body length, true inseam and riding style are important to bike fit, as is where/what you will be riding.

Riding offroad needs more frame/rider clearance than street or park riding.  You should have 4 inches/10 cm for true off-road mountain biking, and 1-2 inches (2.5-5 cm) for road or commuter riding. 

One of the better bike fit sites: http://www.competitivecyclist.com/Store/catalog/fitCalculatorBike.jsp