go 6.5 if your gonna go components. you'll kick yourself for the 5.25" later.
and also, if your gonna do it, spend a little more then you really want to. (you said your looking in the 100 market, i'd suggest wait a little and go for the 200s) the sound will be SO much better.
6.5s will be clearer then your 6x9s. but the biggest problem is... what does best buy have??
i'm not familiar with their product line.
i just put in picasso soundstream components and i'm very impressed with how they sound.