For my Dual monitor set up, i use a program called UltraMon. it manages the two monitors very well. but just so you know, when you hook up an extra monitor you'll need to enable nvidia dualview. it consists of alot of reboots at first, but its worth it. I've been using dual monitors for about a year now, and i honestly don't think i can go back to a single monitor set up, it just gives you so much room to work with in windows. for games, you'll need to disable the 2nd monitor, you can do that just by right clicking on the ultramon icon in the taskbar, or in your display properties.
there is no performance decrease with dual monitors because you have to disable one to play games anyway. however, when i'm in the desktop and browsing the net or whatever, my GPU is warmer than if it was using a single screen.
do it! its worth its weight in gold!!!