Ion Cannons below Mark 3 are not worth capturing. By the time you have one or more the AI's got mark 2 ships (which are basically immune to a mark 1 ion).
Orbital Mass Drivers are also capturable.
Anyway, you can hover over every structure and it'll say "changes ownership on system capture" or something like that (I forget the exact text).
Ions, OMDs, Gravity Drills, factories, fabricators, science labs, distribution nodes...