I have some kind of obsession with really big monitors. I have a 20" widescreen 1680x1050 and a really cheap 19" 1280x1024 which I use dual-head so that I effectively have approximately 2960x1050 of space (times 4, thanks to multiple desktops). But I still find myself wanting more. Sadly for the price I paid for my widescreen monitor a year and a half ago, today I could get a 1920x1200. I'm thinking two of those 1900x1200's side-by-side would just about do it for me. I'm not even sure video cards can support that though; I may as well get a 30" monitor since they can support 2560x1600 resolution, if I go that big. But for the price of a single 30" monitor I could get TWO 24" monitors. This is all irrelevant since I'm not actually going to buy these things any time soon. But I can dream.
You can't beat big monitors for productivity. I really don't even use multiple desktops any more. I just drag things to my other monitor. The more you can see at once, the more you can get done. You aren't reduced to wasting time shuffling windows all over the place. I think that having really nice interface components is the most important thing about whether you enjoy using your computer or not. I'd take a huge monitor over a fast CPU / more RAM/ more disk space any day (within reason, of course). Monitor, mouse, keyboard, in that order of importance.