Bronzite created a 4-page PDF document speculating on the positions of the various Halo installations and how the system might be laid out. Very slick, very professional, and certainly worth a download and a read. Bronzite says:
I started writing a post on the relative difficultly of flushing the Galaxy with the network of Halo installations placed around the Galaxy, but it lead to some interesting places, so I wrote it up a little more formally to faciliate communicating my ideas. I recommend the PDF below to anybody who is interested in the way the Halo Network might be arranged, and what the impact of Installation 04 being destroyed might have been.
Free registration and login required to download the PDF, or check out the HTML version if you prefer (no login/registration necessary).