You have to be aware of the parameters of your amplifier. I checked the link Johnnyboy sent you, and it is correct so far as it goes, except it says that a tube amp puts out the same amount of power regardless of the resistance and that solid state puts out more power to less resistance. That is wrong. They both put out the same amount of power regardless of resistance, just with less resistance the SS will be able to use more of its power, where a tube amp has a transformer that will match impedance.
If you want 8 ohms, you can wire two 4 ohms in series, or two 16 ohms in parallel. The amplifier doesn't know the difference, as the total resistance will be 8 ohms. The advantage to using 16 ohm speakers is that they will absorb spikes in power better, and will be harder to blow. The disadvantage is that you sacrifice efficiency. The advantage of using 4 ohm speakers is the better sensitivity (assuming quality is consistent) and more versatility. 8 ohm speakers are ideal for single speaker or series-parallel wiring, or if you want a 4 ohm load with two speakers. Some people consider a highly efficient speaker to sound harsh. That's part of why vintage Altecs, JBLs, EVs, etc. are desirable for tone.
If you have a choice, get speakers that handle more than your amplifier can deliver. A 50 watt head should have at least a 75 watt speaker, as the head will still send spikes, and you want a speaker that is rated high enough to weather those spikes. If you have a speaker of unknown power handling capability, use the EC code to determine the manufacturer and call them. They should be able to tell you the power handling capacity of the speaker and who it was made for. Then call them and find out where they used it. That is generally very helpful to know how you should use it (or not!).
Hope this helps some!
Gone are the days we stopped to decide where we should go, we just ride...
