Cut said:
If you have the revised box, you don't need SFOS, but you'd still want to get Armageddon, as it has the newest rules (and fleet lists?) in it.
Actually Sky Full of Stars has a good deal of new things in it as well - new ships for all races, planetary assaults, boarding actions, space stations, Sheridan, expanded campaign rules and so on.
To have a the full rules and complete, up-to-date fleets, you need: a boxed game, Sky Full of Stars and Armageddon. Which is obviously quite a lot.
If you just want to start playing, get a boxed set. It contains the basic rules, a fleet book and counters for all of the ships in the fleet book.
If you are patient enough to wait for the second edition, it will be coming out later this year, which will included everything in two books (except counters).
Another route into the game is the starter book (which is only £5/$10) which has a stripped down ruleset and a handful of ships and several Narn vs Centauri sceanrios.
I hope that isn't too confusing.