Virtual,
The ONLY differnece that impedance makes is how much power they can handle. There will ne NO sound difference between an M60 or an M80 at normal to low volume levels. Amplifiers can simply supply more current to the lower resistance loads than they can with higher resistance ones.
If your amplifier is rated to handle 4 ohm loads, you could use a 4 ohm center speaker, and have 8 ohm speakers for your surrounds and mains. You could also use an 8 ohm center, with the rest all being 4 ohms... Mixing and matching speakers of different impedances is perfectly fine as long as the amplified can handle it.
