The factory head unit is 1 1/2 din. If you want to see installed pics of head units, check here: http://impalassforum.com/cgi-bin/ultimatebb.cgi?ubb=forum;f=5
Any model radio will fit in there. You don't have to buy a new bezel to make a double din fit in there. Just shave the one you have. It also depends on if you want to flip the A/C controls and the radio. If you want to do that, then another bezel might be a good idea. Read up in that link.
The factory radio is 1.5 din.
To install a double din radio you will have to modify the factory radio bezel.
For a single din radio, the mounting kit centers the radio and leaves less than 1/2 " all the way around.