I somehow managed to speak with a knowledgeable tech rep from Comcast and the set-up of the new modem was painless. I tested the speed before and after from two different sites, running 5 tests at each site. The average download speed before was ~34 Mbps and ~6.3 Mbps up. The average download speed with the new modem is ~36 Mbps and ~6.2 Mbps up.
That's fast baby!
I bought this modem from Amazon and paid $85, so it'll take me a year to pay for it by not having to pay the $7 to Comcast.
The Motorola SURFboard SB6121 DOCSIS 3.0 cable modem gets the Conrad seal of approval.