I think your best bet is to keep the am/fm tuner that you already own and just add an XM tuner to your stereo. There's no real benefit in having it all in one box unless you've run out of inputs on your preamp. FWIW, I have the polk audio XM radio and it's pretty nice for what it is. I think Crutchfield has them on sale again for 249.
Before you take the plunge into XM you might want to try it out for free on your computer and make sure that you are excited about the programing. If not then I would look elsewhere. As you know, the sound of satelite radio is not all that great and the real reason to get XM in the home is for the programming, not the sound quality. If my local FM stations would shut up and play music then I would have no need for it. Even my public radio station that I have supported for 20 years decided to go talk radio. Oh well, such is life.
Anyway, I would put an emphasis on musical content over anything else. Check all of your options before pulling the trigger.
Good luck.
QE