I think I did a brief write up over on DNY however here's a basic run down.
For your comcast (or any other ISP) accounts setup Gmail to fetch the messages via Pop. This is done in settings > Accounts. Gmail's answer is here
http://mail.google.com/support/bin/answer.py?ctx=gmail&hl=en&answer=21288
If you have domain based accounts such as for KDT setup email forwarders from within your hosting control panel for cPanel it's under Email > Forwarders then be sure to remove the same account under Email > Manage Accounts so that the mailbox doesn't full up on the server and start boucing messages back.
From there you setup Gmail to "send email as", this is done the same way the POP set up is done in Settings > Accounts. Here's Google's how to..
http://mail.google.com/support/bin/answer.py?hl=en&answer=22370
I plan to write up a full tutorial over the weekend with screen shots and all..It's pretty self explanatory though using those links.
