Comcast does not have a cap on downloads. I download gigs and gigs of ISO's and movie files each month with no problems. If your area has the latest upgrade from Comcast your maximum download speed will be 4Mbps and your upload max will be 384Kbps. As stated you will have no limitations on data transfer each month, so happy downloading to you.
That doesn't quite make too much sense because Comcast is now bumping speeds up to 6 Mbps with the option of 8 Mbps by the end of this summer though I really wish they would improve our upload speed more. Why give us that bandwidth then tell us not to use it? It's quite possible certain Comcast markets do that but it still doesn't fall in line with the current speed boost.