The XM on the HK radio will work better if you relocate the antenna.
The NAV on the HK radio sucks and is basically useless, IMO.
If you want full functionality with the HK radio, like navigation prompts through the stereo system, you need to look into getting the HD NIM setup and instead of getting the Garmin 665, get the HD Roadtech 665. Otherwise, you'll need to run the output of the Garmin through the AUX input on the HK. Even with the Roadtech, you'd have to do that to hear MP3's, and the XM from the GPS. Youll have to set the HK to AUX to hear the stuff from the GPS through the stereo/intercom. If you don't care about the prompts interrupting the HK, then the Garmin units will do fine.
There is a thread here that talks about installing the NIM system on bikes that were not equipped with it. It's just something you might want to consider before jumping in...