I love using sketchup and lumion, lately I have ideas of shapes I want to try that I don't yet know how to create in sketchup, more organic type buildings like Zaha Hadid.
I have been thinking of trying FormZ 8 to model with because its something like sketchup but with nurbs tools.
Aaron, you don't need FormZ or any other more robust programs like Maya or Max because you can virtually build anything in SketchUp, including organic shapes like Zaha Hadid's architecture, by using plugins.
Check out the Plugin forum (http://sketchucation.com/forums/viewforum.php?f=323
, as well as the Extension Warehouse and Extension Store (you have to install it) for all the Plugins available. Some are free and others cost a bit of money but it's all worth it.
Some of my favorite are from TIG, Fredo6, Chris Fulllmer, thomthom, Whaat, just to name a few.
If you want to do organic shapes, check out SUbD by Thomas Thomassen (http://evilsoftwareempire.com/subd
Best of luck!
Follow Your Dreams.