The bigger the better is my belief. The more real estate you have the better you can operate smoothly. I have one monitor only for my design layer the other for the OIP, Resource browser, Navigation Pallet, Visualization, and working planes. while also room for my calendar, email, itunes and if I need web browser for research during design.
Granted Apple is not making the 30" any more but the 27" is fantastic.
And if I could add a better card I would run 3 monitors.
Although now I have been spoiled and cant hardly bring myself to use my 17" macbook due to the small screen.
To sum up, money can be tight but 2 monitors is well worth it.