Comcast has offered to update my Modem promising higher speeds, But modem does not really affect the speed. What can be new in the modem then? Can they throttle my activity? Torrent activity?
It is likely the new standards that came out (DOCSIS 3.0 came out in 2006) that your original modem did not support that you new modem does. If you are still paying for a low tier it would not matter if you switched out your modems or not, however if you wanted to move to a higher speed tier by paying more (anything more than 30.72 Mbit/s requires DOCSIS 3.0) you now have the option to do it.
They already throttled your activity on your old modem, that is how they control what speed you get vs how much you pay. As for torrents, any monitoring would happen on the ISP side not the modem side, so changing modems would not change their ability to watch (or not watch) your torrent activity.