Stereo & Electronics - Question about Monsoon system and Ohms

I'm sort of a newb when it comes to car audio, so here's my question. I know the monsoon system is set up at 2 Ohms and most aftermarket speakers are 4 Ohm, but what determines the Ohm-age (if that's even a word)? Is it the speakers themselves or the amp or both?

What I had in mind was to upgrade my system in stages. If I left the stock HU and stock amp, removed the filler speakers in the hatch, and put aftermarket speakers in the doors and sail panels, would the speakers run at 4 Ohms or would the amp make them still run at 2 Ohms?

The second stage would be to upgrade to an aftermarket HU and add a sub and amp. Then the 3rd stage would be to replace the factory amp with an aftermarket one.

the ohmage is the resistance of the speaker. that is the resistance that the stock amp is used to seeing, by increasing the resistance of the speaker it affects the power that it can recieve from the amp.
I believe going to a greater resistance wont hurt the speaker or amp, just performance. going down however is a good way to shorten the life of an amp.