I've never been that impressed by alpine speakers. They are good speakers but for the money, i'd invest in something else. Boston Acustics in the best bang for the buck in my oppinion. ANd i disagree about the 6x9, but that's a preference. I like some rear fill. 6x9s can add midbass, depth but also phase issues, i have my gain at about half of the front, crossover real low- 60hrz, and love it. I've tried to not use them on other peoples recommendations, i just don't like it.
My main advice to save money is go with the cheapest stuff you would want for another 10 years. My system has gone through 3 cars now and still going strong. I skimped on my components, infinity kappas, and they are defineatly a weak link. Then again i got them for $60 used off someone who didn't know what they had. I always want to upgrade but i end up buying other things first, never have got around to replacing. Not that they are bad speakers, many rave about them, but not for me.
Just don't skimp if you feel like you are. Buy the best you can afford. Many people figure, it's better than stock so this should be fine. Then want to go further, etc. Have a clear idea of realistically how far you want to go so you have room to upgrade. If you think you want to replace the 6x9's but wait on it, make sure your amp is 4 channel, simple things like that.
Sorry, long post, not much substance