vApus Mark I gives the opposite view: the Xeon Nehalem is without a doubt the fastest platform, but the latest quad-core Opteron is not far behind. If your applications are somewhat similar to the ones we used in vApus mark I, pricing and power consumption may bring the Opteron Shanghai and even the Xeon 54xx back into the picture. However, we are well aware that the current vApus Mark I has its limitations. We have tested on ESX 3.5 Update 4, which is in fact the only available hypervisor from VMware right now. For future decisions, we admit that testing on ESX 4.0 is a lot more relevant, but that does not mean that the numbers above are meaningless. Moving towards a new virtualization platform is not something even experienced IT professionals do quickly. Many scripts might not work properly anymore, the default virtualization hardware is not compatible between the hypervisor, etc. For example, ESX 3.5 servers won't recognize the version 7 hardware from ESX 4 VMs. In a nutshell: if ESX 3.5 is your most important hypervisor platform, both the Xeon 55xx, 54xx, and quad-core Opteron are very viable platforms.