I can't help you with the 6-disc.
XM radio is completely separate from the radio itself. Both the GM kit and the kit I used plug in with pass-thru connectors.
I highly recommend this one over the GM kit for ease of installation:
http://www.myradiostore.us/xm-radio-...-xmdgm100.html
The GM kit comes with an antenna that requires drilling of the roof. I installed the above in 45 minutes, never having had the dash apart before. Paid $300 for mine a year and a half ago, still works great, a lot cheaper now.
The GM XM kit is around $330 from GMpartsdirect.com.
Note that with either kit connected to the radio you and I have now, XM is engaged with the band button, which cycles from FM to AM to XM.
Hit E-bay for the six disc player. Can't say if it will directly connect to your existing cables or not. I have the same radio as you.
On edit: Says that kit is for '03 to '05. I do not know if the radio was changed for '06, I know that the instrument panel was.
If you're somewhere near MD, come on over and you can have a look at my setup.