I have worked on 98, 98SE, 2000 and ME and am currently working with all of the above.
I see no reason to update from 98 to anything yet... it's still a very viable OS and is fairly stable with a few 3rd party memory managers and other utilities to keep it from hurting itself. ;-)
ME, horrible memory management, DTDs or blue screens of death a good amount of the time when running a D3D game or when simply running windows media player and a sound bit at the same time.
2000, very stable, good layout, looks like win ME but is based on NT technology. No point in upgrading to this for a gaming home system, but definitely a good OS.
-SW