Effective, well, right now, Comcast has imposed a 250 GB bandwidth cap on your traffic. You no longer can use more than 250 GB per account. That is, unless you haven’t already fired Comcast.
I did that a few months back (first taking a speed hit with AT&T DSL, and then a massive speed boost with local SureWest fiber), and haven’t looked back since. I really could only suggest Comcast at this point, if it’s your only broadband option.
And, I’d also suggest you start to move away from Comcast as your TV provider… vote with your wallet. I will move away just as soon as DirecTV launches either HD TiVo (as in, a modern one with home networking) or Windows Media Center support.
Send a message to internet providers that caps are bad, and will hurt them (much more than us) in the long run.