hello, been observing from a far and finally decided to jump in. my ? is this- i have recently been in a brand new buick ranier and was totally infatuated with the seemingly nice radio/gps/xm equiptment in the dash. looks like it'll fit right in. it can't be that easy, right? needs extra something, especially if you don't already have onstar? - please talk me out of this, but it was cool

yes, it is possible.
Grab a beer and read this: http://www.gm-trucks.com/forums/index.php?showtopic=28030
XM can easily be added before or after the conversion.
I installed a NAV radio about 8 months ago and it works great.

There are people on here that like aftermarket units. The pioneer and kenwood are great units. I see nothing wrong going that route. I did the GM unit for two reasons: 1) I like the look of the factory unit (ie no flip up screen) 2) I have factory rear seat dvd and i did not want to lose the functionality of that system.

Installation is relatively easy. The biggest pain is tapping in to the vehicle speed signal, but that is not necessary.

Good luck with whatever you decide.