I cannot answer your question directly, but in our office we too are in the process of aquiring a renderfarm, and because of certain deals with Dell we are getting the dell Poweredge 1850 servers with 2xDualCoreXeons.
This brings me to a question that our IT-guys have asked me: Are there any known issues or pros and cons with running the Windows Server 2003 x64 (r2) OS with either backburner or vray (DR and network)?