typically a home router/firewall/switch by netgear, linksys etc will have wired ports that support 100Mb (million bits) per second for wired, and frequently 1Gb (billion bits) per second.
Wireless g is 54Mb/s, and wireless n is higher (up to 600Mb/s).
What you need depends on... what you are using it for. For internet and online games, your internet connection is much more of a limit than your router. You could have just a few Mb (cable could be say 10Mb down and 2Mb up), FIOS might be up to 50Mb down and 20-30Mb up, for high speed / higher cost versions. All of these are under the speed of wireless g, much less wired.
If you are transferring large amounts of data between home PCs, then gigabit is the best. If not, it doesn't matter as much.
You will not unhook your comcast modem. What you want to do is connect the comcast modem TO to new router. Just make sure you plug your comcast modem into the internet port of the router (the port by itself).
here is a textual diagram:
ISP (comcast) ---> modem (supplied by comcast) ----> Router (the one you purchase) ----> computers and wireless