When I began my career as an assistant architect 12 years ago, I used AutoCAD R12, 3D Studio, CorelDraw 6.0, and Photoshop 4.0 for architectural drawing and 3-D modeling. Today, many architects still use their later versions, but those bulky packages provide many functions an architect will never use. Luckily, there are several open source alternatives that are well-suited for architects -- QCad in place of AutoCAD, Blender instead of 3DMax, Inkscape in place of CorelDraw, and the GIMP as a substitute for Photoshop.
CAD is the weakest link in the open source chain for architecture. AutoCAD provides architects with efficient solutions: you can, for instance, put hundreds of doors, windows, or pillars in your plan within seconds. By contrast, in open source CAD, you have to do most of the works from zero.