Recommendations for a car stereo set-up?

Well, yesterday I purchased a Pioneer 3400 UB head unit (CD stereo)
and a pair of 6x9 Kicker speakers. The person at Best Buy was trying to talk me into getting a separate amp, which I might do after I see how this sounds. So I mounted the speakers in the package tray, now I have to think of how I am gonna mount the stereo under the dash. Should I re-install the three vent factory unit first (factory AC car)? attach to that?

What should I use as a "box for the radio to go in?