Wow! Bad advice for what you're asking for.

If you want an in-dash GPS unit, look no further then Kenwood!

Kenwood KVT-516 DVD receiver at Crutchfield.com
(Or the cheaper/older Kenwood KVT-514)

The navigation unit is made by Garmin for Kenwood. You already have a Garmin as did I.. and how much I loved it I cannot express in words.

It is the EXACT same Garmin interface as all of their portable units, but built in to a nice big screen that hides away nicely.

I'll take a video of mine to show you in a bit, it truly is fantastic.