I would say the Z-5500's and Klipsch have comparable sound, though the Klipsch tends to have a boomier sub and brighter speakers with the logitechs having a more even sound (I prefer the sound of the logitechs overall). The upgrade to 5.1 is pretty significant though, also the digital inputs are great if you use em for double duty from a DVD player or something, but since that isnt a priority, I dunno.
Still your klipsch's arent exactly slacking in the PC speaker department. But if you have a good place to mount the side speakers, surround can be a big improvement (I personally love just having a middle channel to say the least)