if you are going to go BB, they can do the dynamating there too. dynamat is easy, if you passed kindergarten, can use scissors and a hair dryer, you can install dynamat around your speakers. you basically cut out the area in the middle so that your speaker JUST fits into it
then peel of the back, and stick it onto your door like a big sticker. then you sick your hair dryer on it to cure it.
if i were to choose between Best Buy and Circuit City, i would have to say go Circuit City. Depending on where you are, they will have a regional garage that is REALLY REALLY good. I have met a bunch of installers that are highly qualified from there, they train much harder than i expected, but like i said, talk to the installers. and since you ARE going to either a cc or bb, when you go there, talk to other sales reps in other departments that look young, more likely than not, they have taken advantage of their discounts on the stuff, and will know which installer is best, at least at the store. component wise, i would have to say that circuit city has better stuff, higher quality, and some of their installers know custom work, hell I was one for a while ^_^. i also worked the bb route, talked to other installers that worked there and left pronto. right now, bb carries rockford fosgate amps/speakers. if installed correctly they will sound pretty good. circuit city has MTX amps and speakers that sound good, and as always, the bottom line of any particular brand will still sound like crap, so stay away from the Road Thunder MTX, and the lower end PUNCH amps and such from RF.
At the chicago auto show i got to talk to the installers of the Rockford Fosgate Demo vehicle (nissan truck, i think it was a frontier) and was the only one allowed to check out their instal tee hee (insider connections kick butt). It sounded pretty nice. I competed with MTX for a while, before moving on to ID. ID has to be my fav as of yet, but they can only be found in true hard core car audio stores, neither bb or cc carry them at the moment. but IF i was constrained to stick with either of the stores and the stuff they carry, i would have to vote cc hands down. i would also look at the polk audio speakers and such while you are their (BB doesn't carry polk either). Polks have very nice sound, and are very controlled speakers, i love their voice i used to have polk db separates all over my car ^_^, they are the predecessors of the polk momo series, and have little relation with the dx series past cosmetics. the dx series still sounds great, but the db/momo sound sometimes wants to make me melt. as far as headunits go, they will be pretty much the same at cc and bb, with cc having a better selection, and higher quality stuff, such as the kenwood excelons and the crazy expensive alpines. I would also warn you not to get any of their double din stuff. they look cool, are insanely expensive, but have crap for processors. high end single din head units tend to sound much better, and are of better construction. you will be able to hear the pioneer the other boys were talking about head to head against the kenwoods i was talking about at CC, and i don't think that BB carries both.
i hope you are happy with your decisions with what you buy. if you need any other advice feel free to email.