My setup is this: a MacBook (13") with Apple's 20" Cinema display.
Why do I mention this? Because I think it is very close to the perfect setup for developing wep apps. Why? Because of Parallels, which allows me to use Windows XP (or Vista) on my MacBook while I'm running OS X. This makes it possible to have windows running in full screen mode on my Macbook display, and OS X on my Cinema Display. This makes cross browser web development a lot easier because, after I've developed my code to work on Safari, I can open up IE in windows and tinker with things, watching both IE and Safari/Firefox at the same time.
Note: to do this I highly suggest getting more than the default 512 mb of RAM for the MacBook. I have 1256 mb installed right now (which makes the Macbook fly doing most things) but I'm going to upgrade to 2 gigs once I need to be using Windows more.