The issue is x86/x64 being a turd. Throughput of just about everything drops when you go over the 36Gb boundary for some reason. I have no idea why. It can drop by as much as 30%. This is from experience operating VMware on the physically the same type of kit (except with FC SAN / NetApp).

I'd personally rather buy high end SPARC64 kit which has linear performance scalability but we all know what happened to Sun, plus it doesn't run Windows anyway :( whimpers a little