Is your existing router not doing something that you NEED for it to do, like additional Ethernet ports or reaching areas of the house that it currently doesn't reach? If it's to increase speed, forget it. You can't go any faster than your connection no matter what cable modem you have and you can't go faster than your cable modem no matter what router you have.
What you CAN possibly do, if you are using the wired connection for your internet access, is increase your wireless speed because your Xfinity modem router is probably not terribly capable.
What is the exact model of your current Xfinity modem?