Yeah, the best way to go is to go through Circuit City. Here's your step by step guide.
1. Go to Circuit City.
2. Grab the SB5100 Motorola modem and a Comcast self install kit (useful for the high quality RG6 cables and splitter).
3. Go find someone and tell them you want to activate service.
4. The deal should be that the modem is around $80 minus mail in rebates which makes it free. They also should give you a $75 CC gift card. The service is $19.99 for 6 months. Not bad...
5. Take the modem home, plug it in, make sure it syncs up to the cable line, check your computer, and see if you're online. Comcast used to have you manually call in the MAC ID addresses but now they "self provision" the modems. Not sure how it works, but you should be online if your cable line is good once you plug it in.
6. If your modem isn't going online but is syncing, call them up and ask them to manually provision the modem so that you'll get online. If the modem isn't syncing, it's probably a cable line problem and you'll have to call them if you don't know what you're doing to fix it yourself.
7. Either way, call them and make sure that they have you on the $19.99 for 6 months rate code, many times they forget about it.
It comes out to a free Motorola (the best) modem after rebates, a $75 Circuit City gift card for signing up, and a special promotion of $19.99 for 6 months on service. The service is then $42.95. I use Comcast right now and am very happy.
Good luck, this is the best way to get Comcast. I hook up many clients through this method and the incentives are nice and Circuit City is the best place to go to get it done trouble free. Good luck, post if you have questions.