https://mydeviceinfo.xfinity.com/ will walk you through finding which devices your local service + speed work with. Spoiler: most DOCSIS 3.0 modems are compatible.

Buy it, unplug your existing modem, plug the new one in, and call Comcast. I had mine out of the box and up and running in all of 15 minutes (most of that was spent on hold and waiting for the modem to reboot).