A place to start with replacing stereo in '07 G6

So I'm loving my 07 SE.

Factory deck is base , no mp3, no XM, no 6 cd changer.
The sound is very impressive for a factory.

I took advantage of my extended warranty plan with GM and had the back speakers replaced *cough*made a funny sound*cough* need to be replaced *ahem*
So that's done, sounds great.

Thing is, no mp3 capability is killing me. Xm and 6 cd changer I can live without but I really like the info center stuff for mileage, temp and oil & trip stuff etc.

I've seen multiple posts that some say you CAN upgrade, some say not 100%.

What are the options available to me? I imagine if an upgrade is possible from GM that it might cost an arm and a leg right?

I also saw a post or two that mention installing an input for an ipod or mp3 player? What's the success rate on that? Feasibility / cost?

Sorry for all the n00b questions right off the hop, just wanna get my baby sounding the way I want it too

Thanks for any info or help!!
T