You are right. Actually, I need to virtualize these servers as a temporary solution.
In fact I must restructure about 50 or more domains. Domains are based on different Windows Server Solution (Windows 2000 Server, Windows 2000 SBS, Windows 2003, Windows 2008 etc.). Restructure include transition from stand-alone machines to failover clustering solution based on Windows 2008.
Also, restructure must be done with existing physical machines. Except SANs, there are no more new machines. All these machines have productions roles. I must find a solution to do this transition with less impact possible.
I think that DCs must work on virtual machines no more than 2 or maybe 3 days - Time necessarily for transition.