I think you really need to decide your true objectives before a real recommendation can be made.
When most people want to just use internet for their TV viewing, it means they want to cancel their cable or satellite subscription, yet you are considering paying Comcast for television service to get a cable card...why wouldn't you just use that service that you're paying for?
There are ways to do things, but it would be a huge PITA if you're going to pay Comcast anyway. If you're paying Comcast, just use their feed. Get a Roku or Amazon Fire or AppleTV for Netflix, Hulu, etc., but don't waste your time running XBMC to try and get what you're already paying Comcast for.
An antenna is the easiest way to get your local channels if you don't want to pay Comcast. TiVo or a ChannelMaster DVR+ are great ways to add DVR functionality to your antenna.
Most legal apps (ABC, ESPN, HBO, etc) require a cable or satellite subscription to login and view the content, and not all subscriptions will get you into all the apps.
Are you looking for easy, cheap, free, quality, what?