ambient occlusion is a very cool technique. It's what we are using here at work at the moment. The only thing with ambient occlusion is that it is VERY slow to render. We are lucky enough to have 1000 procs to run it on but on a single proc machine... it's gonna take a while. There are some ambient occlusion shaders written for mental ray so that might be worth looking into.

here's a link for you too:
https://www.andrew-whitehurst.net/amb_occlude.html

As for the FG GI thingy, I just use final gather at the moment when I'm at home. I get pretty good results with just that. I havent really tried the GI option as of yet.

Alan