I haven't seen any (official or recent) guidelines that discuss what you can or can't run gazebo on.. The User Guide has the statement below that many have echoed before:
Depending on what kind of environment
and robots you're simulating, Gazebo
can be computationally intensive. The
exact CPU requirements will depend on
how you use Gazebo, but in general we
recommend that you err on the side of
having a more powerful CPU.
Gazebo can be run on a single processor machine.. but be warned, expect laggy performance.. especially if you have complex objects in the environment. Most people would probably agree that at least 2 processors would be desirable, and 3 gigs of RAM would be workable. Also, Gazebo is decoupled into gzserver and gzclient.. but the operating system manages how these processes access your computer's resources.
Don't forget that you have to leave "space" for your control code to run, and if you process camera data or pointcloud data.
Finally, the graphics card is an important piece of hardware to consider as well. In the old days, Nvidia cards were better supported, and more people appeared to use those, but I've run Gazebo 1.8 on virtual graphics cards and I have been content.
