I just started using two screens on my PC about a week or so ago. Even though this is something that my geek friends have been doing for a long while, I was a bit set in my ways, and didn't want to mess with another way of working or get sidetracked by too many things in front of me at once. But I finally decided that having documentation, email, IM, etc. open in one monitor and the stuff I was working on in another was a good idea. Sort of like having a book open in front of you actually... it's very nice not having to resize stuff to fit as much. And if I need to test something in MS Windows, I can just Remote Desktop into my other box and use it on a full monitor by itself, again, without having to flip windows back and forth.
X-Windows uses this thing called "Xinerama" to spread the display across the two screens, otherwise the displays are completely separate and you can't drag stuff from one to the other. It works, but it's a bit odd sometimes, for example, screen shots are now massively wide images of both screens at once, and wallpapers need to be sized to fit both screens (or it stretches). So I just ran across a link in LifeHacker to a site with a bunch of cool wallpapers called InterfaceLIFT. Tons of images at 2560x1600 - perfect for my new setup. (Actually, perfect would be 2560x1024, as that's what my monitors have for resolution combined - 1280x1024 x 2 -, but hey, it works). Some are really fun - I attached a thumbnail of the one I decided on above. (I like planet themes, I've named my projects after the new Dwarf Planet classification... :-) ) This one of the Golden Gate Bridge is pretty great too.
But would you believe it, just as I start getting used to two screens, Diego goes and gets a three screen setup at work. 7000 pixels wide... Insane!