I have phone and Internet and a modem for each. I purchased my modem called MC and they setup the new modem to handle the internet and the old one they demoted to only take care of phone.
I did what billh did, I still have the eMTA for the phone, I split the coax and run internet through my own cable modem.
I'm getting 12.5 down and 1.25 up, so I guess the eMTA is doing OK for now. Perhaps if I had the "gamer" package (which I don't anticipate at this point) I'd need a separate modem?
My understanding is that for anything >20 you should have bonded channels which means DOCSIS 3.0 modem at a minimum...Of course anytime Metrocast wants to issue an eMTA with a DOCSIS 3.0 modem would be the best route because then you have an all-in-one setup.