This has been my experience with dual-head, your mileage may vary:
Most games, particularly if they are running full screen, need control of the keyboard and mouse. Depending on the game and how it was coded, you may or may not be able to move the mouse off the side of the screen into another application. Clicking outside the game's view usually results in the game minimizing. Also, games running full-screen on one monitor tend to do weird things to the other monitor. Particularly if the game is running at a different resolution than the monitor normally runs at. I have seen all the windows on the secondary monitor be "pushed" sideways, for instance.
In summary, you can't do much with the other monitor while a game is running full-screen. You pretty much have to alt-tab out to get access to the windows over there, which means you don't have a great advantage over only having the one monitor. However, you can leave a walkthrough or something open over there, if that's what you're after.
Dual head is a very useful configuration to run... I really enjoy mine. But there aren't very many benefits where gaming is concerned.