If you're interested to read about Microsoft IT's migration of www.microsoft.com to the Windows x64 architecture, then check out their white paper. The paper is actually written by the operations team for MS.com. Here's an excerpt from the conclusion:
After completing the transition to the x64-based platform, the memory limitations that the operations team had been struggling with for years evaporated. The resulting benefits of the elimination of the memory contention problems will allow Microsoft.com to continue to set the industry standard for availability while greatly increasing both current end-user performance and future capacity. The new platform also enables the next wave of 64-bit Web applications written in ASP.NET 2.0 by means of the 64-bit-capable .NET Framework 2.0. The performance of these applications running in a native 64-bit environment is expected to be even better than that of 32-bit applications running on the x64 platform.