I just ordered a set of Infinity Kappa 603 component set. I read that it comes with trim rings to fit in oversize GM vehicle stock locations. Otherwise you would need to make your own baffles. Not too hard I would think. I'll let you know when I install mine. I was looking at boston acoustic pro or ralley, infinity 605, and mb quart RSD before this deal came along. I've heard some running on deck power and amp. I say go with an amp.
Just go to all the stereo shops you can and listen to what they have. It's best to bring your own cd though, something you listen to mostly. I didn't do that and I know some songs sound better on some speakers. Some salesman put in some 80's crap and maybe it was the music that made them sound bad.
Pick whatever you like. You're going to be the one listening to it. Most of all do your homework. Check out sounddomain.com and see the reviews on the forums.
I am also a diehard Boston man myself. Even though I can get the polk momos through circuit city for cheap as dirt, I had to pay the price for the pro series. I wont put any other mid/high in my ride...until I can afford some rainbow references!