Ohm is a measure of impedance. The lower the number, the more efficient the electronic device. That'* why you get more amps and lower ohms.
If you put a 4 ohm speaker into a 2 ohm system, you get what is called an impedance mismatch. Since the the speaker isn't as efficient as the system, you lose bass, volume, and clarity. Since it'* such a minor mismatch though, you really can't tell the difference. You can just turn up the volume a bit, and problem solved. Based on this I don't feel it'* worth the extra cash to match the Monsoon system.
If you are replacing ALL the speakers AND the radio though, it would be best to remove the Monsoon amp and replace it with a regular 4-channel amp. You will lose a lot of bass doing this though since the 6x9s in the back are no longer "subs" because the Monsoon amp also acts as a crossover. Easy fix though is to buy a 10" sub and a 500w monoblock amp and there ya go.
2004 Chevy Silverado Z71 - 15% tint, FTDRL, All-on-high, 5000k lows/fogs, tinted tails, color matched grille, Sound by Kenwood.