I don't know why I haven't posted this before.
I know people often yearn for faster cpus, more memory, bigger hard drives, etc.
Nothing has even come close to the increase in enjoyment and efficiency in working with a computer for me than having 2 monitors instead of 1.
Windows handles two monitors perfectly, and it requires no special setup, other than having a video card which can handle it (ati and nvidia have best drivers for it).
If you only have 1 monitor, and you're looking to improve your computer experience, i can't recommend more highly setting up a dual monitor system. Nothing comes close to it in my mind and i'd rather lose my right arm than go back to a single monitor.
This is even doubly true for anyone who programs, or has certain programs that they tend to run minimized to save desktop space (you can simply leave it running on one screen while you do work on the other).