How many monitors is too many?

My main computer has 3 monitors - 2x 19" and 1x 17".  Now I find myself thinking that "hey, a fourth would be nice."  Anyone using multiple monitors that's found a point where another really doesn't help that much? 

Typically I'm setup with Outlook & iTunes on one, a code editor in the middle, and a web browser testing stuff on the right.  Or if I'm working on photos it's Bridge on the left, PS in the middle (which is also the only calibrated monitor), and the PS toolbars on the right. 

The thought came from the fact that the 17" is also used as a secondary monitor for my MacBook and the primary monitor for my fileserver - it's on a KVM switch.  And when I use it for one of those two computers it seems limiting to "only" have 2 on my main computer. 

Had another thought.  Get a smallish (~20") flat screen TV that takes a VGA input and use it both as a TV and a 4th low-res monitor when the need arises.  I wouldn't be doing much on it other than iTunes, videos, or actually watching TV so the resolution wouldn't be all that critical. 

I'm an optimist and I'm just waiting till I can plug the computer directly into my head.

It a trick question . . . the answer is one!

that's interesting Marian, I can't think of a practical use since we have browsers with tabs, but it's pretty cool looking.