Make sure your ISP offers 56kbps connections. 56kbps connectivity may have been available for nearly two years, but many ISPs still don’t support V.90 services. If yours doesn’t, it will matter whether you’re using 3Com’s x2 specification or Rockwell’s K56flex specification, since these are not inter-operable. 

Choose an easily upgradable modem. V.90 is a new technology and further optimizations to enhance performance must be expected. Your modem should be upgradable through the flash ROM, with easy-to-find downloadable updates available at the manufacturer's website. 

If you're planning to buy an internal modem, you'll have to choose between ISA or PCI-based models. Generally, PCI modems perform a bit faster, but the deciding criterion will probably be the type of free adapter slots your PC has. Newer PCs will have four or five PCI slots and one or two ISA slots, and vice versa in older machines. Since PC manufacturers are planning to phase out the ISA bus, it would be a wiser choice to buy a PCI-based model if you want to use it for any length of time. 

The modem speeds claimed by the manufacturers for their products may not be what you get in reality. One factor that can affect speed is your modem's firmware and drivers. Performance will depend on just how new the controller code and the drivers are. Another factor is that modems that have a controller chip of their own will probably perform faster, since they don't tax the CPU. Software modems will use less power, but will lose speed. 

And finally, your modem's chip set also matters. While the V.90 standard was developed in order to end the incompatibility between x2 and K56flex, modem vendors tend to upgrade an existing technology rather than develop an entirely new one. Hence, if your V.90 modem originally used, for instance, x2 protocol, you can expect it perform noticeably faster when connected to an ISP's modem that also originally used the same protocol. 

Now, let's get back to our review of the 3Com US Robotics Courier V.Everything. (Don't you love saying that?)