I'd suggest you check out a
Motorola 6121 or
Zoom 5341-J either one is about $75. When you get your new modem just hook it up and call Comcast, they will activate it for you.
They are both are DOCSIS 3 modems which can provide you more speed and stability. There are a lot of factors that effect your speed including: What speed tier you are subscribed to, your router, you computer and the quality of the signal to your modem. (You can check signal stats here: http://192.168.100.1)