Well, in a nutshell, yes it does matter which modem you get. Some ISPs support only a selection of modems, others support a more variety. Check with your local ISP to see what brands/models they support.
I have used and experienced 4 different kinds of cable modems and their speeds were not affected by any particular brand. I've dealt with Motorola, Toshiba, Linksys, and RCA modems and all seem to be quite on-par with features and speeds.
When you are setting up your system for a cable modem, I HIGHLY suggest you use ethernet to connect the modem to your computer, rather than USB. Not all modems support USB hookups, but that's no big deal.
When using USB with my particular ISP and the Linksys as well as the Motorola modems, I was able to reach a peak of only 750Kbps. This is still quite fast, but when I used ethernet, I was able to fully utilize my "whopping"

800Kbps connection.
I hope I've answered your questions and have given you a few pointers to look out for. Enjoy and happy surfing!