Infinity speaker replacements

[mcs219]

I have a 98 Grand Cherokee 5.9 Limited with a 180W amp and Infinity speakers. One of my rear door speakers seems to have blown, and I'm considering replacements. I was never disappointed with this system, so, in the interests of saving money, should I get another infinity speaker from the dealer, or an aftermarket? Anyone have any suggestions for good aftermarket apeakers that cost ~$50-$70/pair? Also, how do I match a speaker power rating to the 180W amp? I was told to go a bit lower, say ~150W max power rating so that the amp will punch the speakers pretty good. Is this true? Thanks.