The following is math that I'm sure Merc is aware of, and I don't mean to sound patronizing in spelling it out.
4096000 pixels (30") vs. 2304000 pixels (24") vs. 3840000 pixels (current 22" 4:3 * 2 monitors)
So a 30" has 178% the space of a 24" and 106% the space of my pair.
The questions really are:
1) Does more pixels give you more productivity above a certain point?
2) Do more monitors give you more productivity than more pixels?
I'll tackle #1
90% of programs aren't appreciably better above 1280*1024 (web, word, e-mail, media players, etc.)
Some programs, however, love more desktop space (spreadsheets, Photoshop, AutoCad, certain games)
Games where higher resolution allows more information to be displayed (Supreme Commander) or where more detailed information is beneficial (any FPS that supports sniping and a long view distance) can give you a distinct advantage.