Beginner Stereo setup questions

[Jecht]

crutchfield sells replacement tweeters from infinity, kenwood, and a few other if you dont want to spend teh money on a full component set.

Well, I want to upgrade the stereo anyways. It sounds bad. I just need to pick a cheap but decent amplifier and subwoofer, and I need to make up my mind if I want to power the speakers with the amplifier. If I only use the amp to power the sub and leave the speakers powered by the headunit, will I have any trouble connecting the speakers to the amp instead of the headunit later on?

Edit: One more thing - there is a sub/amp combo on crutchfield that I am looking at.
http://www.crutchfield.com/S-GngCIMg...50&I=700V103XT
The amp is a 2 channel. I know I'm a noob @ stereo stuff and I want to keep the left and right channels separate to make sure it really is stereo when I hook the speakers up to it eventually. Will there be any problems with this?