well, if Nissan wants $339 + $250 (total $589) for the install of the factory headunit with the XM receiver, then my suggestion would be to get an aftermarket headunit with an XM receiver. You can get them cheaper than the Nissan unit and IMO you will get a better quality headunit.

The XM Commander I think uses an FM modulator to get the sound through your car's speakers. This method reduces sound quality.

I don't know about the PAC for the '05. I know there was a LONG thread about it, but I didn't really follow it since I don't own an '05. The idea behind the PAC is using the cd-input of the factory headunit as AUX inputs. I don't know what the outcome of the discussion was for the '05.