I recommend the Boston Acoustics. I have the Rally 5 1/4 components up front, and they're awesome! The highs are great, very crisp and very bright. And they won't break your bank - the Pros however will

You can get them for like $225 I think for the front set or less. Go to your local Tweeter and listen to 'em, I'm sure you'll like them. As far as an amp, I don't really know which ones are best, but I've heard Alpine and Eclipse make good ones. For the back speakers, are you talkin about the 4" coaxials, i.e. for a 2-door right? If so, I just threw a set of Boston FX-4 coaxials since I dont' really care about the sound in the back (especially on the convertible) but they do sound really good for coaxials and were only like $60 bucks shipped from Ebay where I bought them from. I did my whole setup for $825 installed and that's with a JL 10" sub, Kenwood amp, the Rally's up front and the coaxials in the rear. It sounds great and bumps and didn't cost an arm and a leg. Hey if you make it down to my BBQ and/or HIN you can have a seat in my 318iC and listen to them firsthand in a Bimmer - especially if you get me that hookup on the HU!
Thanks again bro, hope this helps
Sal