^^^ Do what Jim said! ^^^All those systems with the tiny 2.5"-3" midranges will probably sound tinny to you now, so if you like your current speakers perhaps it's a better idea to just keep those and get yourself a decent subwoofer. The Dayton SUB-1200 is really the only contender in your price range, and it's a nice sub to boot.
You will need to do more than the Dayton speakers - to beat your Optimus ones.Are Dayton speakers as good as their subs? I found these
I'm extremely skeptical because of their price though...