Nat,
In laymans terms, if you were to buy a new 7.1 receiver, you could use a mic to balance out the db level of differnt mfg. speakers, but you could not "timber" match them. The tweeters and midranges of different mfg's may not blend correctly, thus giving you inferior surround results compared to a matched speaker setup. Go slowly if you are upgrading your system. Start with a new receiver. HK is a great place to start. Give yourself a budget, and let us know so we can direct you to the best "bang for the buck" based on your listening preferences. Then you can decide if you need a new speaker setup or not. Have fun. There's a ton of new equipment on the market. Rear centers help the "enveloping" effect of surround sound. The newer dvd's use 6.1 and are very impressive with the new receivers.