I installed a Pioneer DEH-P3100UB this weekend. I'm using the stock speakers and no amp at this point, but the difference in sound is still incredible. The best thing about this is that I can just plug directly into my iPod and I'm good to go: charges the battery and allows me access to my playlists.
I'm impressed with the pioneer so far, and the price was great. Installation was a relative snap with some splicing and a few color-matching issues, but it all came out well.
I'll have pics of the actual install later on tonight.
UPDATE: Here's Cognito's dash:
My GPS is a Mio Moov S401: a very capable and cool little device that can be hacked to run basic Windows CE apps (which I've done).