Power @ 1 or 2ohms is substantially more, however your amplifiers efficiency will drop quite a bit and it will run much hotter. I run my high end amp @ 4ohm gives you better sound quality in your trebile where its needed. When running a woofer this isn't quite as important so I usually drop my Ohms to either 1 or 2 depending on the capabilities of your amp.I have an isolated amp for each purpose though. If your using only 1 4ch. amp just stick with 4 Ohms
Normally the Ohms is the resistance offered by any electronic device and is definitely required, the more the resistance the better the efficiency and better life of your equipment, what Steve has told is also true on the other end, however it needs to be seen that no resistance on the other end would act like an open circuit, this could be fatal to your equipment too, now you have to choose what kind of drivers you would like to install in your car. hope this helps.
what mrsq says is correct, however you have to know that the lower the ohms the more the distortion..not to forget the risk of tosting the amplier would be multiplied to a greater extent too...hope this information helps
It is a gamble. Get a good amp that likes to run 2 ohms and you shouldn't have much of a problem. E.g. old school Xtant amps love running at 2 ohms and run cooler at 2 ohms (in my experiences).