I am mulling over whether to buy a new monitor, to go along side my current setup of two 24 (ish) inch monitors. What I want to know is whether this is worth the money (probably around £200)?

I think most of us will agree that two monitors is much more productive than one when programming and developing (Jeff Atwood has said this many times on his blog, and I imagine that most of you are fans of his), but is three much more productive than two? What I'm worried about is that I will have so much space that one monitor will be used for things that are not related to the task (music, facebook etc.) and it will actually make me less productive.