for speakers, think used. You should search the archives here for recommendations in your budget range. Now and then you can find Vandersteen 2C listed for ~$400 or so. Some older Paradigm may be a good value too (I have an old pair of Paradigm 7se that are quite nice considering they're not worth much). I'm sure there are others worth considering. If you get floor standers you'll want to shop for something here or on ebay that you can pick up locally. I wouldn't get a center channel speaker now - just set your receiver to "Virtual center". You can get a way with using your yamaha for rears, but you will want to consider replacing them with speakers of the same brand as your fronts eventually. If/when you add a center channel speaker it's critical that it matches your mains - more critical than it is to for the rears to match. This should be a consideration when you purchase your mains. Otherwise purchase your mains now with the intention of selling later and replacing them with a full matched 5.1 system.
Finally, if you haven't subscribed to the Audiogon Bluebook, you should. It's a great research tool that's worth the cost.
Finally, if you haven't subscribed to the Audiogon Bluebook, you should. It's a great research tool that's worth the cost.