Wireless routers typically support one of two wireless standards, Wireless-G or Wireless-N. Wireless-N is newer and Comcast recommends that users with Blast! or Extreme XFINITY Internet service upgrade to a Wireless-N router. While you can access the Internet with all routers regardless of your speed tier, a Wireless-G router may prevent you from getting the maximum possible speeds.

Note: To create a wireless home network, you’ll need both a router and a modem, or a gateway that functions as both (see below).

The following routers are Comcast-approved:
Netgear WNR1000 (Wireless-N)
Netgear WNR 3500 (Wireless-N)
Linksys WRT310N (Wireless-N)
Netgear WGR614v8 (Wireless-G)

The following routers are Comcast-approved, but are older models with limited support:
Netgear CG814v1 (Wireless-G)
Netgear CG814v2 (Wireless-G)
Linksys WCG200v1 (Wireless-G)
Linksys WCG200v2 (Wireless-G)
Linksys BEFCMUH4 (Wireless-G)

Source:
http://customer.xfinity.com/help-and-support/internet/upgrading-outdated-networking-equipment

http://customer.xfinity.com/help-and-support/internet/home-networking-equipment-checklist