Switching from XM to Sirius - Help!?

XM radio was pre-installed in my Infiniti Fx-35. It sucks, so I want to switch to Sirius radio now, but apparently i need to do something to my radio to get Sirius to work. The dealer pointed me to www.soundgate.com but I'm not sure what I'm looking for on the site? Any ideas what equipment i need to make the change?