i just got a 07 G6 GT and was thinking about the same thing, i wanted to replace the head unit altogether with an in dash with nav all in one, i found this one that i kinda like, and you can set the lighting to red so it matches the rest of the interior lighting on the dash:
[ame]http://www.amazon.com/JVC-KW-AV50-Touchscreen-Receiver-functional/dp/B006Y0Y9MU/ref=sr_1_5?s=electronics&ie=UTF8&qid=1339634247&sr=1-5[/ame]
i do have ONE cocern that im trying to figure out, and that's if whether or not the steering wheel controls and Driver information center will be effected, cause i dont really want to do that.
Good Luck! message me if you find anything we can bounce ideas back and forth