QIIME has already documented the process of installing QIIME/VirtualBox quite well. The following video also helps clarifies things.
When choosing the amount of RAM, make sure to choose at least notch or two left of the green/red line.
At 3:30-How to create shared folder
Erin has started this short guide to QIIME. Our hopes are to join the information on our sites in the future. This hypothetical awesome website will also have information about MG-RAST, PC-ORD, etc.