well, you didn't ask me, but i've evangelized this everywhere and done it a couple times, so i'll chime in.
nowadays, with any operating system, its pretty much automatic. (though more recent ones are better of course; like say, win xp. apple has done this forever though.) it *can* be done with just two graphics cards (and in that case, its usually best if they're of the same type.) its easier if you get a double-headed graphics car. in that case, the setup utility provided by the card manufacturer will provide some thing (e.g. let you choose how you want the monitors to function: under windows, mine, for example, lets you choose to have both monitors display the same thing, or one to display only video, or one to display a "magnified" version of the first, or - the usual config - the two to be combined into a single large monitor. in that case you get to specify their physical orientation (is one next to the other, or on top of the other).
then, inside your regular setup, you'll be provided with the typical options for the resolution of the screen that you'd like to choose. in this case, its pretty simple: instead of 640x480, for example, you'll just be given the option of 1280x480. windows would treat it as a single physically larger display.
some programs are not well behaved with dual monitors, and there is some setup that has to be done via windows or monitor card utilities (e.g. where do you want new programs to pop up - its annoying when you have a program on your right monitor and its always popping up dialogs on the left monitor) and so forth. but in general, its not hard to get used to working on one very large desktop, and with only a few exceptions (windows being the least well behaved here - versus mac or linux, e.g.) its just like having one very large desktop.