Here's some information and benchmarks from Anand's latest server benchmarks. Looks like maybe Nehalem isn't all it cracks up to be, depending on what tiles and scenarios of software it's running on...
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.
Multi-Core Upgrade Guide
Do you want a world ranking? Join Team Fusion in Forum Warz 2011!