Ever played a game with more that one screen?
It's a very expensive thing to do, mostly because you have to pay twice or three times as much on screens. I have two 23" FullHD displays and I'm quite happy with, depite their individual problems (One has no DVI and the other suffers from extreme backlight bleeding and a dead pixel) as well as the problems involved in dual screening them (The one is an LG and the other is a Samsung, go figure).
The first time I ever played a game with two screens was Command & Conquer: Tiberian Sun. I had an old GeForce 6600, with nVidia's horizontal and virtical spanning. This useful feature turns all your attached screens into one big one, allowing some programs to maximize across all of them. Tiberian Sun was one of those games with an ini file you could play around with. It was easy to change the width and height of the screen to anything you could imagine. e.g. 2560x1024. I only had CRT screens back then so my head hurt a lot having to look at two of them.
In my opinion, the kind of games that benefit most from multiple displays are strategy games. The extended view of the battlefield makes a huge difference. Older games such as Age of Empires and Command & Conquer had fixed 2D pictures so that the higher the resolution, the smaller the pictures on the screen, and the more of the battlefield you could see, so the poor guy with the crappy computer (yeah, one which could only run AoE at 640x480!?) or only one small screen, had a disadvantage. Most of the new 3D games don't allow this sort of exploitation, as a change in res just means more pixels for the same-sized object.
Supreme Commander, my second dual-screen gaming experience. Now this game is serious about it's dual-screening capabilities. It uses any other display connected to the computer to display an extra view of the battlefield. That plus the ability to partition screens allows incredible control over what's going on. Also an extreme advantage over the guy with the 15" CRT in the corner over there, who probably couldn't afford a good enough graphics card to run the game smoothly at even 800x600. lol. A good thing about this game's dual-screen capability is that it does not rely on ATi's Hydravision or nVidia's H-Span, so I don't have to use funny tricks to get it to work. Bottom line here is that Gas Powered Games really hit the spot with this one! If you have two screens you should definitely try it!
Burnout Paradise interested me in it's option in the configuration screen allowing up to three displays. In excitement I hooked up my other screen and selected "2" to see what it looked like. I was disappointed at what I saw. All it did was make one super-wise view and squish it into only one screen. It dawned on me that it required nVidia's horizontal spanning , which my new HD4000 did not have (damn you ATi!). So I checked it out on my friend's computer and it didn't look all that great anyway, so no real loss.
Determined to get horizontal spanning on my computer I Googled all over the place and found some interesting piece of software called SoftTH (Software Triple Head) which sort-of does the same thing a Matrox TripleHead2Go does but for free (instead of R3000+). What is boils down to is that I can do what nVidia can do, but with my ATi. Sure it has a few problems, but it works great! The latest 2.0.1 Alpha version is the one you should use, all the 1.x ones are old. Get it here.
Below are some of my experiences (And yes the saturation is higher on the left monitor, I only discovered after I took these screenies that it had been changed in CCC \) :
Burnout Paradise. My first run with SoftTH gave me what I thought would happen, a car cut in half in the middle.
But after changing the configuration to use 2 displays I got it right. As you can see it's no fun unless you have three.
Counterstrike was a funny one. Look where the crosshair is!
It obviously was not designed to be played this wide...
HAWX would have cut the plane in half but I had to see what it would look like...
...but all I got was... nothing... :(
Ok so SoftTH is still in Alpha so you can't expect everything to work as planned. But it's some really awesome software and it allows for some pretty wicked screen setups:
Render resolution is 5888x2100!
I wish that was my desktop, except for that little screen at the top, it looks a bit weird.