I have a dual 22" setup and find the second monitor very useful for referencing materials when/while working in full-screen on the first... saves a lot of time with window minimising/maximising.
I don't play games that much, but a second monitor is great for keeping an eye TV (news, sport, etc) while doing something else... saves having to go into the loungeroom to see items of interest, sometimes missing stuff cos I'm too slow getting there.
As for playing games, I would imagine the game content itself would be best on just the one monitor, with related and non-related stuff on the other... as has already been said, the monitor frames would likely be a distraction. While going from one screen to the other may be instantaneous, the frame could well be the difference between shooting an opponent's spaceship (whatever) and not.
