A 30in seems to be pretty good to me. I can have two full sized firefox windows on the top, or one window and like 2-3 notepads for coding, and then all my IMs run along the bottom.
Try it and you will see. Its great, there are so many things you can do.
Can be coding on one, and looking at the work on the other. Or have your IMS on one while you work on the other.. etc etc.. Once you have two screens, having one seems almost inefficient.