Wires connectors, Banana on the receiver end and Spades on the speaker end, but Banana - Banana is fine too. I just like spade connectors on the speaker ends only because it looks cleaner with the way the wire come off as oppose to the bananas.
Center is your next step for HT, then I would go sub, then surround speakers.
Subwoofer, get the 12" if your budget and space can accomodate it. For movies, there is no such thing as too much bass!
BTW, don't think you have bottom of line speakers, really doesn't matter, what matters is that you gonna have HT experience!
Enjoy!