When were talking about impedance of speakers with regards to matching to amplifiers it can be important, Especially if were talking about speakers rated at 4ohms and below as they will require a much more capable amplifier. Most speakers are 6 or 8 ohms and generally will work with most any reciever or amplifier. Just to be on the safe side, and to be sure you are buying a quality reciever look for those that have wattage ratings at 8,6 and even 4 ohms so you can be sure they will work.
Its not like if you take a 4 ohm speaker and hook it to an amp not rated for that, that everything will go up in a ball of flames the instant you turn it on but at loud levels or longer listening sessions the heat can build up and be a major problem or more current will be required of the amp than can be provided. I have driven 4 ohm speakers on amps that were never meant for that type of load just fine, Do i reccomend it however? no.
There is not a formula, at least not one that would tell you anything relevant when it comes to matching speakers and amps. The bottom line is, get a good reciever and you'll likely have no problems at all. If your talking about getting 6 ohm speakers all around vs say just for the fronts then you should make sure the reciever can handle it.
As far as wattage is concerned, You dont really have to match things up, Most recievers are rated at 110 wpc for the fronts or around there and this is more than enough for all but the most demanding listeners with large rooms or power hungry speakers. The wattage ratings on speakers arent a particularly useful specification, more than likely its there for people who dont know much about audio systems and for them it can be a major selling point.
