From my experience, go for Creative or Logitech speakers for 5.1,6.1, 7.1 surround sound systems. Provided you have a fairly decent pc AND you either know what soundcard you have, or you have put 1 in yourself. Any top soundcards made by Creative\Turtle will do.
You should consider how large the room is, what kind of music you will be listening to, how much you want to spend, what range of sound you want (deep bass which may require more speakers), along with where you will place the speakers.