One topic that comes up frequently with respect to capacity planning is the consideration for virtualizing one or more farm servers. 

Technet has a Virtualization Planning page for SharePoint Server 2010 that is a good starting point for this. 

Some basic considerations for virtualization in SharePoint 2010 include:

• For Hyper-V, use Windows Server 2008 R2 or Microsoft Hyper-V Server 2008 R2
• The R2 versions have advanced support for live migration, VM storage, networking
• Ensure your hardware is Hyper-V compatible to take advantage of hardware-assisted virtualization
• Plan for at one external physical Domain Controller.  Other DC’s can be virtualized.
• For high availability considerations resources should be physical not virtual.
• For production environments, 4 cores of your CPU is the minimum recommended starting point
• If feasible, leave SQL on physical hardware.  If not, split SQL Servers to no more than 1 SQL VM per host

Also of interest in planning Hyper-V is the Hyper-V Planning and Deployment Guide.   While not specific to SharePoint 2010 recommendations, the underlying understanding and recommendations are great to be aware of in architecture design.