Honestly,

I have a high end 30" HP monitor and another 32". I used both for quite a while but unlitmately I just use the one high end 30" now.

Couple reasons,

1) I run with the 1 large window and 4 tiny window setup and it's easier to supervise your clones even if the windows are smaller than being on a large screen next to it.
2) My perfomance is better. Running two monitors prevents me from using SLI. Even if you dont have SLI it will be the same.
3) It's easier to take action on your clones for short periods if you dont have to drag your mouse pointer over to another monitor
4) With Keyclones PIP window swapping I make up for the clone windows being so tiny I can't read the chat print. It flips between characters pretty fast so it over comes the disadvanteges all together for me.

I doubt I will ever run more than one monitor again unless Keyclone gets hit by a bus. :S

Never would have guessed I would have liked 1 monitor better but now I play movies on the second one