We are looking at setting our 2005 enterprise box and, Dell 2950 x64 blah blah blah. We are looking at using a HP DL360 as a witness server and have the mirror be a VM running on ESX for failover. I noticed a post that the witness server should be the same OS as the principal and mirror, but cannot find and reference elsewhere. We are looking to run 2003 Server R2 enterprise x64 on the 2950, 2003 Server R2 Standard x86 on the witness server (DL360) and 2003 Server R2 enterprise x86 on the ESX Server. We will have SQL 2005 Enterprise on both the 2950 and ESX box, and SQL 2005 Express on the witness box. Does anyone see any problems with this.