I just love my current monitor setup - with three killer Dell Ultra displays (2x Ultra 20" with 1680x1050, and 1x Ultra 17" with 1280x1024).

And I'd easily slap down a few more pesos for a fourth monitor because:
1) I can. 2x EVGA 8800 GTS video cards makes me feel like it's Christmas all the time ;) and handles 4x DVI monitors out of the chute.
2) I'd have an obscene amount of real-estate (as I'd spring for another 20" and have 3x 1680x1050 + 1280x1024 for a whopping 6 megapixel desktop)
3) It would just look killer on my desk
4) There would actually be a few times every month where I would actually be able to use all 4 monitors at once ;) 

But I'm NOT going to pony up for that 4th monitor just yet because: while 4 monitors would be melt-your-face-off cool, it would also be melt-your-face-off HOT. Yes gang, that's right: 3 sets of 'ultra bright' cathode ray tubes and millions and millions of happy little transistors cook the crap out of my office. In fact, I frequently have to turn on a small fan on my desktop to get a gentle/quiet breeze flowing between me and my monitors. Without that I sometimes feel like I'm out getting too much sun on the beach. In fact, it's about 45 degrees outside. I've got the heat in my office turned off, and the window open. But it's 80 degrees in my office right now.

I know... horrible problem to have, but it does bug. Cuz I'd really like that 4th monitor ;)