There isn't a lot of traffic on Projector Central for speaker specific questions. You may want to try reading up the articles at Audioholics and asking on their forums. You may be able to use the existing Onkyo receiver and get some decent speakers, but it's important to go to stores and listen to as many speakers as you can to get some idea of what may be out there.

I couldn't believe the jump in audio quality I had going from my original Kenwood cheap speakers to my Definite Technology speakers on my 'cheap' A/V receiver. So much quality depends on the speakers, and they can be very different from each other.