Here is a list of modems comcast has approved for use. You can look at their D3 modems.
The SB6120/6121 are the most popular I would say. Keep in mind some of the modems on the list are not for retail sale and can only be rented from comcast. Like the cisco dpc3000 modems, you can only rent them from comcast and can't buy them anywhere. A google search shows that they are selling the dpc3008 in retail now though. Others are digital voice and internet, you can't usually buy those either or activate them.
http://mydeviceinfo.comcast.net/
I would still go with the motorola SB6121. If you are on a budget, I think the ubee or zoom modems are cheaper, but not sure on their quality.
Most modems can bond 4 downstream and 4 upstream channels that gives 160mbs download speed (theoretical), there are newer modems that can bond 8 downstream channels giving 320mbps downstream. One of these modems is the dpc3008 (not sure if the dpc3000 does 8 channels). However, I don't think comcast has even enabled 8 channel bonding yet, and you are only on a 25mbps plan, so it doesn't really matter to you.