It is recommended by most industry experts that you use all matching drivers, meaning in a perfect world the actual drivers in each of the speakers is identical (make, model, sensitivity, impedance.....) since that is most likely not to happen for most people, I would suggest at least matching the front three, since they will be the most noticable, if they do not match. If you use your exsisting speakers for your fronts, and get a new center you will have a high likely hood of a non tone matching front sound stage. The example I like to give is the scene in Terminator 2 when the Harley comes across the front three speakers, well if you do not have matching fronts and centers, it will sound like a Harley from the left speaker, a honda from the center, and back to the Harley in the right speaker. In a 5.1 or higher surround sound system, the encoded sound that comes out of the left and right speakers will carry the "off screen" information and the center speaker carries the "on screen" information. Most would also say in a 5.1 system, it is best to have the surrounds on the side walls (dipole/bipole) beside or slightly behind the listening postion, and about 18" to 24" above ear level.

My 02 cents worth.

Trey