What Navigation system ?

I would highly advise you to make the decision as a whole, in other words, same brand head unit and nav system.  Otherwise you will miss out on a lot of the little features you probably want, like the music muting when the nav system announces, etc.

I know Pioneer now makes an all in one headunit/nav system that looks pretty cool.

http://www.pioneerelectronics.com/pna/product/detail/0,,2076_3151_64850099,00.html

I would think downside to that is you couldn't watch a dvd while using navigation.

I have this unit now and it's awsome!  It has "memory navigation mode" so once the route is set you can eject the map disc and watch a DVD.  The downside is not all functions are avialable when in "memory mode" like setting new destination (it will still reroute you if you miss a turn), or looking up POI's.  For that you have to put the map disc back in. It will however, remember where you left off in the movie and resume there when you go back to DVD.  You can also set a "home area" like a 10 or 20 mile radius and it will store that in memory all the time.

It's kind of a pain if you need to access map disc a lot (frequently add destinations or looking up POI's).  It doesn't bother me too much, but I have a 3 y/o daughter who is not very patient with me when I have to stop Winnie the Poo, so I'm installing a DVD changer to solve that problem.