I've installed Windows2000 Pro (OEM version, final).Since my graphic card is Matrox Millenium G400Max, therefore I use Matrox 5.00 Beta drivers.Flight Simulator 2000 works very good under Direct3D, even better than under Windows98SE. On the other hand, Flight Unlimited III doesn't work with Direct3D and doesn't work under Glide as well.
Totally, Windows 2000 would be the best platform when all the drivers will be mature.
Adam