The one study supporting one big screen is of bad quality, the studies supporting two screens are of medium quality. The former study was done by Apple, it's small and people were doing only one task (Spreadsheets); the latter ones had a bigger sample and people did various tasks. A survey over the productivity-blogosphere revealed people are mostly in favor of two medium screens over one huge screen. Bill Gates has multiple medium-sized screens, so does his close subordinates.

I have two bigish ones (23'', 1920x1080), just in case. I consider this was the most amount of utility per dollar I got in the last 2 years, excluding money spent on relationships. I intend to buy a third bigger one (33'') soon.