I guess it really comes down to how much you want to spend and what you're looking for feature wise. Were I looking at doing architectural stuff seriously, I'd probably go with LightWave, as that's what I'm using now, and its a pretty robust package right out of the box.

It can handle architectural stuff well enough as is but for more ease of use on architectural stuff there's LWCad. I've not had the chance to try it out, but from the looks of it, it pretty much adds some of the tools, that would be missing compared to something like AutoCad.

Obviously, try any and all demos you can get you hands on. I've added two links, one to LWCad, and one to a forum, that is based heavily on architecture, and taking a look and asking around there might help you with your descicion some. Really, tho I'd think any and all of the major packages can handle architectural stuff. I know LightWave can. As they've mentioned above, I've heard good things about Sketchup as well.

LWCad  -  http://www.wtools3d.com/index.php

3D Allusions - http://www.3dallusions.com/

 

Core i7 950@3.02GHz | 12GB Corsair Dominator Ram@1600mHz | 2GB Geforce GTX 660

Lightwave | Blender | Marmoset | GIMP | Krita