If your technically (IT) inclined you could go the DIY route. I got tired of the limitations of the off-the-shelf nav/entertainment units so I researched alternatives and landed on Centrafuse:
http://centrafuse.com/
I shoved a mini PC in the dash and connected two 8" touchscreen LCDs. Centrafuse has a pretty solid forum community and group of plug-in/skin developers. It'll do NAV, AM/FM/HD radio, DVD/Bluray playback, games, OBDC, multi-zone support, and more.
It's a project in and of itself...but once you get it up and running it blows any other currently available NAV/entertainment system out of the water.
NOTE: Even if you go with a boxed product, be careful which interface product you choose to connect to the wiring harness. I tried a couple different ones ('cause I wanted to keep my chimes too) and every one either failed after a few months or exhibited a hi-pitched whine when the motor revved...most are junk. I got a straight up GM harness connector (just wires...no box or pre-amp) and linked directly into the harness. I lost the chimes...but 'oh well'..it's worked great for the last three years...no whine...no problems and it's actually louder than the pre-amped units.
I can get you more info if you on any of this if you need it...just let me know.