Depending on your needs, you may want to seriously consider a Rosen DS-GM1010-P11. It's a plug-and-play, touchscreen, OEM-look, DVD/Navigation head-unit. You can get them for $325 (refurbished) on the Rosen Outlet site. These head-units are very easy to install, don't require any adapters and retain all factory functions (steering-wheel controls, warning chimes, XM, OnStar, etc).
I have quite a few posts on here about this unit, including a full review. Very powerful unit and it looks like it came in the car from the factory.
EDIT: Here is a link to a thorough review:
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