do you plan on upgradeing to better speakers later on?
if so may as well shell out the bucks now for a amp that has a good amount of power. the worst thing you can do is be underpowered with high end aftermarket speakers.
i think the stock front speakers can handle maybe 50W's i don't know about those rear speakers. i guess you can just turn the gain down on the rear speakers and you'll be good to go. i wouldn't push much then 75w's to the stock speakers.