Win 95/98/ME vs. Win NT/2000
First, Windows ME (Millennium) is not the same as Windows 2000. Windows 2000 is the new version of Windows NT 4.0. If you don't know what Windows NT is then you don't want Windows 2000. Windows NT and Windows 2000 are primarily for business users and has increased security. Windows ME, on the otherhand, is the new version of Windows 98, and is intended for the home user.
Windows 2000 is not designed for games, its emphasis is on stability and security. If you are wanting flexibility, ease-of-use, and a the ability to play the latest games then you want Windows ME not Windows 2000.
If you upgrade to Windows 2000, you are stuck. There is no way back to '98 or ME. Windows 2000 uses a format other that FAT32 called NTFS. Windows 98 nor ME can read NTFS so all of your files will be lost when you try to move 'back' to 98 or ME. Unless, of course, you have a friend with Windows 2000 and the space to store your files for you.
Windows XP is the newest version of Windows from Microsoft. It is a merging of their home version and business versions into one common base. Most people, will be wanting the HOME version of Windows XP. Windows XP Home is the new version of Windows 95/98/98SE/ME line of operating systems. Businesses or those interested in a bit more from their operating system will want Windows XP Pro.