This is really important check out the SharePoint Team Blog @ http://blogs.msdn.com/sharepoint/  for more info.

Buy 64-bit Capable Hardware

At IT Forum the SharePoint team shared information that this was going to be the last 32-bit version of SharePoint.  This was reconfirmed in the WSS 3.0 SP1 and SharePoint Server 2007 SP1 overview article.  You'll see more and more of this message incorporated into the content.  All of the peformance and capacity planning content on TechNet on both WSS 3.0 and MOSS 2007 was based on 64-bit systems.  If you are purchasing new hardware, it is recommended by Microsoft and the various hardware vendors to buy x64 systems to "future proof" your investments.  Although SQL supports Itanium and x64 for performance and scale, SharePoint supports x64 and will continue to support this hardware standard for SharePoint servers.

64-bit Windows Operating System

Although you can run 32-bit OS on many 64-bit systems with emulation.  With less than 4GB of RAM it's true you won't see much difference between 32-bit and 64-bit SharePoint server roles and in some test cases it may seem faster with 32-bit.  For environments where more than 4GB is needed, 64-bit OS and 64-bit WSS or MOSS 2007 is recommended.  With Windows Server 2008 deployments we recommend running the 64-bit version fo the operating system as 64-bit native support in Windows Server 2008 will provide better scale and better performance.  What about ifilters? More and more vendors are addressing the needs of the 64-bit Index server.

Multi Socket and Multi Core

Tests done by the SharePoint perf team, HP, Intel, and validated by testing done by the solution accelerator team in the beta model of the SharePoint Capacity Planner suggest that going from single processor to single processor with dual core nearly doubles the throughput/requests per second.  Going with quad core in multiple sockets is even better :)

Memory RAM is important.  More often than not we see customers not using enough RAM.  SharePoint IIS 7 Worker processes do need RAM.  It is a good practice to start with 4GB of RAM.  In recent discussions with more detail in the storage whitepaper you'll see recommendations for 8GB for large environments on the SharePoint WFEs, Query, Index and 16GB on large SQL Servers.  More details in the storage paper: Performance Recommendations for Storage Planning.

-Ivan