I have Infinity Reference 505cs component'* in my front doors and they sound awesome (of course the Pioneer GM fit DeH-P77DH helps clean up the sound too over the factory Am/FM/CD/7eq radio). For extra tweeters do what I did -find a 1997 -newer BOSE equipped GM car in a salvage yard and get the sharkfin tweeters that fit up high by the 1/4 window to give a little more of a surround sound with decent highs for $10 (that'* what I paid for my pair). I found mine in a 2003 GP GTP. Also, right now Infinity reference components on ebay are going really cheap. Check there.
Oh, and T. If you need a hand putting the stuff together let me know. either you can come to Sauk Centre here (I have access to a heated shop) or I can come up to Alex. I've monkeyed with the stereo system'* in these Bonneville'* more than a few times and would be happy to lend hand.
P.*. If you get and after-market speakers and aren't going to amplify them go to Best Buy (only place around here that I can find that has speaker adapters) and get speaker wiring adapters so you don't have to cut your factory wiring (makes it easier to go back to stock when you sell the car too). They sell for like $5-$7 per pair. If all your speakers (rear 6"x9"*, front 5-1/4"*, and tweeters) you will need 3 sets. Still under $20.
If you want I can also help locate some nice stereo equpment (just give my your price range and I'll try and find some nice stuff).
Anyhow, just give me a shout if/when the time/need arises.