I would recommend that you remove the old 28K modem before trying to install/use a newer modem. However I would really like to recommend that you read the articles about the Geoport modem here on the website as well, since people have also gotten 6500 and beige G3 modems to work in their 6400's as well.
I have tried with no success to get an external USB modem to work using a USB card in a 6400 computer. It is very tricky to find a compatible USB card for the 6400, especially now when all of the cards are USB 2.0, you will need a USB 1.1 compatible and the 6400 is notoriously picky about what USB cards will work. However I think it might be good to get a usb card for use with a USB printer so that you can use something fairly modern instead of being stuck using an old apple style serial printer.
I recommend installing a PCI network card like a realtek 8139 chipset and using cable or DSL internet instead if you can afford it or if it's offered in your area. this will work much faster and is relatively easy to find and set up on your computer.
