Yeah, some z-5500 sets connect with RCA and some have speakerwire connections.
If you have a speakerwire version you should be fine hooking up some 8ohm speakers.
Something to note though is that the Logitechs are going to have an internal crossover that's pretty high compared to a stereo/hometheater setup, so you're not going to get the full benefit of upgrading the speakers with the lower frequencies they could provide compared to the Logitech satellites.
There's no substitute for getting out to some stores with some familiar material and demo-ing some different options to see what kind of sound you like.
I went over some general options in this thread in my long post (it's a work in progress)
http://forums.anandtech.com/messageview...atid=42&threadid=1778632&enterthread=y
Depending on what you get, it could be a big jump in quality.
It's certainly a good option if you want to move up to a speaker/receiver system eventually and want to take it in steps.
Are you looking for bookshelf speakers I'm assuming?
Some options might be
below $100
Polk R15s / R30s on sale from outpost.com
$100-$200
Infinity Primus
Paradigm Atoms / Titans
Athena
JBL Northridge series
$200-$300+
Klipsch
Boston Acoustics
Axiom
Ascend Acoustics
Onix Rockets
Swan
Aperion Audio
B&W
(I'm not totally sure on the price divisions here, but it's roughly right)