All servers do not use Xeon's. The last two servers I built used dual P3 1Gz processors in an Iwill dual proc board. I personally have experience with Win 2K Server. Very easy to use and supports multiple users, multiple domains, PDC and BDC structures, Active Directory and is VERY secure. It is also very tailorable to your needs when it comes to domain structure, security permissions etc.
I guess what you use would depend on your needs.