Anytime you have subs and tops, you need a crossover somewhere to make sure each cabinet gets what it should. That can either be in the mixer (some have sub outs), in a separate box (either a crossover unit or a driverack type box), or in the speakers themselves, as some powered units (usually the subs) have internal crossovers. Which one you use depends a lot on the speakers you pick.
If you go with the same manufacturer, then the built in crossover (if it's there) will probably work. If you go with a different manufacturer's sub (and that's no problem and often done), the internal crossovers may have specs that might not mesh well with the other product, so an external one might be more appropriate as it would give more control over things. An electronic external crossover (like the dbx DriveRack PA+) will give you features that aren't in the manual external crossovers or the built-in ones, including delay and feedback reduction. Delay might be important if you go with a single sub, as normally they are placed away from tops and a delay to either the sub or the tops might help get the sound a little cleaner. Things to think about and research (or ask more questions about).