I would have to say that gaming across three monitor is something i was hesitant on doing for a while as the cost was high, but also whether i really needed to.
I then decided to build myself a new computer just for gaming, no bloteware, and just stick the games, as such I got myself a GTX590 and decided that gaming across three monitors is something i'd really like to do.
Now let me be honest, gaming across three monitors (again my opinion) is great, and once you've gamed with three, you could never go back to the single monitor, as it just feels, as if you're missing out on something on the other 2 side monitors.
With that said, even when gaming you don't tend to even look at the other 2 monitors on the sides, it just gives you a more immersive feeling while gaming, and adds that little extra to your gaming experience, but again you don't tend to keep looking at them.
My experience with games such as BF3 over 3 monitors has been great, and has in some occasion saved my bacon, where i was able to see the enemy just in the corner of my eye, in one of the side monitors.
If you're looking at increasing your gaming experience, I would definitely go for it. If you're a casually gamer, and browse website more than anything, then running across three monitor, might not be the best thing, and you could spend the money on extra ram or a new graphics card or something.
I hope this helps,
I create a few videos a very long time ago about one of my computers. I hope these give you a little insight.
All the best mate, and happy gaming!
http://www.youtube.com/watch?v=6EJ75fa3lo0&list=UU8hYoLoZq8Mkm63WLHoJffg&index=6
http://www.youtube.com/watch?v=kPCD5WGkSU4&list=UU8hYoLoZq8Mkm63WLHoJffg&index=9