ohms questions

[Automission]

When buying an audio receiver is it better for the receiver to give out more ohms that the speakers? Say you have a 5.1 setup, all 4-6 ohms. Would it better to have a receiver give out 4-6 ohms, or should it be higher, like 6-8 ohms? Is there a limit to how many ohms you can put into the speakers?

Also what does the wattage mean, like if the speakers are 150w, should the receiver give the same amount out, more or less?