They currently cap at 250GB per month.
Currently, there's only one plan for most residential customers unless the new higher speed network has been rolled out in your area. If you have the new network available, then you may have a few tier choices.
Also, "Power Boost" is less spectacular than advertised. It currently only affects up to the first 10MB of a download and the first 5MB of an upload.
Comcast FAQ about "Excessive Use"
Acceptable Use Policy
What are the new Internet speeds that Comcast will offer with the launch of DOCSIS 3.0?
With the launch of DOCSIS 3.0, what are all of the speeds that Comcast High-Speed Internet offers?
Is the new Comcast High-Speed Internet available in your area?