Don't buy jumperless peripheral cards, whether of the newer variety called "Plug'n'Play" or the older kind that requires a DOS utility to poke registers in the card at bootup time. These will cause you no end of grief. Plug'n'Play isn't yet supported under Linux as I write, and it would be totally nasty to have to boot DOS first every time you want to run Linux. (A lot of these cards don't even hold their settings across a warm boot.)

You need to be extra-vigilant about this when buying. The tiny, reptilian brains of most computer salespeople cannot seem to encompass the existence of clone-box OSs other than DOS or Windows. They think boot-time setup utilities are just fine and jumperless cards are the best things since sliced bread (easier to set up and a whole fifty cents cheaper too!). So they'll push jumperless cards at you with glee and abandon. Foil them, or you'll suffer for it later.

Don't buy so-called "WinModems" or anything that advertises "RPI" or "Rockwell Peripheral Interface" on the box. These are ways for manufacturers to save a few bucks on firmware at your expense; they won't work without driver software that runs only under Windows.

Don't get stuck with a 2-button mouse. Specifying a three-button model is an easy detail to overlook when filling out your order.