WinME is MS's last attempt to create a win9x based OS... and failed miserably.
Win 2000 pro is a workstation-style OS based off of the NT kernel. In fact most people "in the know" refer to it as NT 5.0.
Windows 2000 pro is Windows 2000 server with the services and some configuration tools stripped from it. So it is slightly faster and more secure out of the box, since it has less crap running on it then the Win2k server version would have.
Their is no plain W2k or W2k home version. Pro is the bottem level. WinME was designed for home users. W2k Pro was design for corporate workstations.
Stay away from WinME.
People like to beat up on it, but it's like a kid who gets bullied. So you feel sorry for it and chase the bullies off. Then you turn around to see how little poor WinME is and he spits in your face and punches you in the balls.
Then runs off laughing.