It's a question that comes up often. Personally I like working with two or three 30" monitors in sitting down use or 2 x 24 x 2 over-under for bench use, but spacewise 23-30-23 is good as well as it'll just fit on a typical desk - and it's all about how you compartmentalise your working space.
My CAD-related setups are either 30-30, or 24x4 + external master vis.
Despite the apparent size of the 30-incher, you still can't get a definite separation of windows as you would with two 23/4'ers with many task-effective window sizes and for many types of working, two of those can be more effective than a 30-inch (where you'd be using Expose a lot otherwise, or coaxing windows into different configurations).
However, of course you can cram more info onto a single screen on a 30-inch. It just depends on what you're doing.
I think for a general domestic user, a single 30-inch might be better as you can for example put movies full-screen on it, etc. If this is a work tool, you might like to have a longer think.