See Linmodems.org for more information on Linux-supported winmodems. These modems cannot be upgraded under Linux. A small number of modems come with DOS software that downloads the control program at runtime. These can normally be used by loading the program under DOS and doing a warm boot. Such modems are probably best avoided as you won't be able to use them with non PC hardware in the future.
CardBus modems are usually winmodems much like PCI modems.
Your best bet for now is to find a card that lists compatibility with DOS and Windows 3.1. Fax modems need appropriated fax software to operate. Also be sure that the fax part of the modem supports Class 2 or Class 2.0. It seems to be generally true for any fax software on unix that support for Class 1.0 is not available. An exception to this is the Linux efax program which supports both Class 1 and Class 2 fax modems.