From Wall Street Journal about screen size and prodcutivity study (funded by NEC :P ).
"People using the 24-inch screen completed the tasks 52% faster than people who used the 18-inch monitor; people who used the two 20-inch monitors were 44% faster than those with the 18-inch ones. There is an upper limit, however: Productivity dropped off again when people used a 26-inch screen."
It basically says that you get less productivity from two 20-inch screens, than one 24-inch one. Presumably, since productivity dropped off at 26-inches, a dual 24-inch setup wouldn't be that great either. Of course, it all depends on what kind of work you do. :)
"People using the 24-inch screen completed the tasks 52% faster than people who used the 18-inch monitor; people who used the two 20-inch monitors were 44% faster than those with the 18-inch ones. There is an upper limit, however: Productivity dropped off again when people used a 26-inch screen."
It basically says that you get less productivity from two 20-inch screens, than one 24-inch one. Presumably, since productivity dropped off at 26-inches, a dual 24-inch setup wouldn't be that great either. Of course, it all depends on what kind of work you do. :)