And Win2K = WinNT5?
And how stable is WinNT4?
And how unstable is WinME?
And let's say I am moving to WinNT instead. What are the main pros and cons of moving from Win9X to WinNT?
1. Yep.

2. Very stable. In all my years of using/trying to haX0r() an NT network, (about 3 years.), I have NEVER had an NT box crash on me. EVER. NT4 is one of the most stable OS'es iv'e ever seen, barring Linux. Speaking of which, here's an interesting fact: Did you know that NT has only 4 more security holes/bugs than Linux? (NT 34, Linux 30)

3. WinME is VERY unstable. I'd go as far to say that it suX0rs . In short, it's a waste of your time.

4. Moving from WinXX to WinNT shouldn't be much of a problem. It's the little things, like DirectX, and games that cause problems. If you install the lastest release of DX into an NT box that has all the Service Packs installed, you should be good to go. Also, make sure your software will run on WinXX/WinNT before even thinking of switching to a NT/2K. I think you can find TONS of good reasons to switch, if you ask anyone experienced w/ Networks, eg. Myself, and probably a lot of other people here at VBWorld.

I'd like to add something myself, too. Win2K is awesome!