Both ethernet and USB can go way faster than your cable connection can. If your hookup is slow, I'd suggest calling your provider and complaining; it may be that they made a configuration error somewhere or can tell you how to tweak things. Or, maybe the modem is bad. Also, is it slow all the time? As I understand it, cable will slow down during peak usage times.
Also, what is your definition of slow? Slow, as in web pages take forever to load? Now, thats bad. But, if slow means it takes several minutes to download the latest Springsteen album, that may or may not be so bad.
If you want, you can try the speed tests at
DSL Reports. I imagine they'll work with a cable modem setup, but I don't know since I've got DSL.