To drive a triple monitor setup, unless you are willing to settle for some ancient Matrox video card that supports triple-head, you are going to need two video cards. In the case of Nvidia cards, getting two cards and using SLI isn't really an option because SLI limits the system to a single monitor only (although recent drivers have enabled a second display but only in Windows Vista or up, XP users have to disable SLI to get more than one monitor to work). I'm not sure about the limitations of ATI video cards and dual/triple monitors though.
The performance of the hardware is largely irrelevant these days, most cards support two monitors right out of the box. Don't even bother with three monitors for gaming, since multi-monitor support for games is almost non-existent outside of a few flight simulators. Personally I use my dual monitor setup mostly for browsing the web on one monitor, while keeping other smaller windows open on the other monitor, such as a music player, instant messenger windows, full screen picture previews, etc. I also often open one windows explorer window on each monitor so I can easily arrange files by dragging and dropping them between windows.
When it comes to watching videos, it all goes to the primary monitor which allows me to keep track of something else on the secondary monitor if I want. But for the most part watching video takes all of your attention so there is no need for a second monitor. The second monitor comes in most handy when you are using the computer for almost any multi-tasking project. Just watching video or listening to music wastes a second screen, and virtually all games only use one monitor anyway.
As for the monitor dimensions, widescreen monitors start to quite a stretch to look all the way over at the far edge of the second monitor, fitting three at once would require turning your head rather a lot to see the extremes. I honestly would not use a triple monitor setup unless the two extra monitors were 5:4 or 4:3 aspect at most. The center screen could be wide, but if the other two were also wide it would start to be more of a pain in the neck (literally) than it would be worth.
One thing to avoid is the so called \"1080p\" monitors that are becoming more and more common lately. They run a native resolution of 1920x1080 and a 16:9 aspect, the manufacturers call it \"full HD\" which they somehow advertise as being better than the more standard 16:10 screens that run a full 1920x1200 resolution. The most important rule to remember here is that the wider the aspect, the smaller the monitor is at the same diagonal measure. Meaning a 19\" 5:4 monitor has a larger screen area than a 19\" 4:3 or 16:10 monitor, and larger still than a 19\" 16:9 monitor. A 24\" 16:9 monitor is actually barely larger than a 20\" 4:3 monitor despite what the manufacturers would have you think.
And last but not least, if you have to ask what you would do with multiple monitors; you probably don't need them. Of course even without the need, one can very quickly adapt to the use of multiple monitors and it becomes a need...
But wanting something doesn’t mean it’s what you need 
Basically I did the same as you did Krom and kinda miss it. I sometime also using applications that require some more screen space and then I need to work with windows over windows which I don't always prefer to do.
