The ohm rating refers to the load of resistance on the speaker/amplifier. The higher the ohm rating on the speaker, the more power you will need to make it loud (more resistance at the speaker = more power to run it). If the speaker resistance is too low (i.e. 2ohm or 1ohm) your amplifier will have less resistance, hence more power being delivered to the speaker.
NOW TO REALITY:
have 2 ohm speakers isn't a big deal, as long as your amplifier can deliver power at 2 ohms (many of the decent or more powerful amplifiers can sustain 2 ohm loads, but you need to check). Don't think that any amplifier will do it. You may blow the fuse (instantly), or worse, the amp will heat up and blow (most likely more damage).
So, if you replace the factory 2 ohm speakers with 4 ohm speakers, you are going to have 1/2 the sound output from those speakers (twice the resistance = 1/2 the sound) - not a very good idea. The reason infiniti has this setup is to get the most out of the speakers with minimal power (saves them money).
ohms is the resistance of flow for electricity. you cant "change" the amp from 2 to 4. its capable of powering in either way. its the way you connect the subwoofer/s up to the amp. it allllll depends on wiring.
yea im looking for a good yet cheap set of components and possibly hook them up to an amp. I have a kicker L5 on its way (should be here monday) and i wana get something to compliment the highs and mids