I run 3 monitors off my main rig, and it really isnt difficult to set up at all. My primary and secondary monitors are hooked up to my ti4400, with the tertiary monitor connected to a pci voodoo banshee.
The easiest way to go about it is to use a dual head agp card (all but the cheapest models allow you to run dual monitors nowdays). Most will have one vga and one dvi output (dual vga cards are harder to find), so you'll need a dvi > vga adapter to run two CRT's. Some dual head cards in that configuration will come with an adapter.
Another option is to add a pci video card (or two).
Once you've got your multiple vga outputs, then it's a simple matter of connecting monitors to them, enabling them in display properties and adjusting their placement to the primary monitor.
If you're using windows, suggest using 2000/XP as 9x can have some problems with dual monitors (although it's not impossible). I also reccomend using something like ultramon
. It's not free, but if you've used multiple monitors without it, you'll find it's well worth the price.