Motorola SurfBoard 6121. Works excellent with my Charter HSI. Speeds received are well above what I am rated at with Charter Internet Express (15 Mbps) and running speed tests at several different sites (speedtest, speakeasy, dslreports, etc.) have me getting download speeds ranging from 25-42 Mbps and uploads just a hair over 3 Mbps, for what that's worth. What it may actually be, I don't know other than it is significantly faster than what I got with AT&T DSL Elite.
It does sound like the key factor with HSI is to make sure you get a DOCSIS 3.0 compatible modem.
You might want to also check out DSLreports.com as they probably have a Comcast forum. They have one for Charter issues and there was a ton of great info there. Other site to check is avsforum.com as there is a lot of good high end networking info there regarding media playback. If you just want to get into the nitty gritty about modems, routers or anything very technical with networking check out smallnetbuiler.com. Probably the most in-depth site I found (as linked in the sticky in the forum here on [H].