Fax abilities I suggest you get a modem that can fax. It is hard to find one that doesn't so don't worry. It doesn't add anything to the price, at least I think, because I have never seen one that can't fax. By the way, faxing only goes up to 14.4K, so you don't have to worry about standards. Brand This is actually an important decision.
Name brands are marginally more expensive than non name brand.
On the most part though the name brand modems perform better than most generic ones. Get a brand name one, unless it will cost you a lot more than want it to. I put off buying a 56K modem for as long as possible. I owned a 28.8K modem and thought that the 56K wouldn't be that much faster. I was also skeptical of the standards.