What version of windows do you have right now?
winME is a win98 3rd edition basically. Not really worth the money since you can download most of the upgrades from MS for free. but if you really feel the need for a new OS, ME is the way to go for games and light use.
Windows 2000 is a robust, NT based OS for buisinesses. It is quite stable for a windows OS. The problem is device driver availability. Many devices, such as my ATI TV wonder, do not have win2k drivers yet and would be useless in a win2k system.
Hope that helps!