The modem shouldnt effect speeds.. Most modems are speed rated pretty equally, but in other terms of docsis 1, 2 etc, that may be a different story. Changing the modem may change stability and behavior of your connections, but the speed will not (shouldnt) change , since its still going to be using the same config file from your ISP.
I agree. My old modem worked well when it worked, but I don't think it could handle the (then) recent speed increase. Shortly after Earthlink went from 3 to 5 megs, I started having connection problems and the modem was hot to the touch. I could still get 4.9 meg DL's when it worked, though. ;)
The new Ambit modem is considerably cooler and works just as well.