This is information I wrote up previously, but it seemed logical to put the information in its own easy-to-find article instead of in an Adaptive Processing Server article.
For more information on sizing your SAP BusinessObjects Business Intelligence 4.0 environment, visit Sizing SAP Solutions on the SAP Service Marketplace (S-ID required). Go to Solution Life-Cycle Management -> Hardware Sizing -> Sizing Guidelines -> Solutions & Platform – New Structure -> SAP BusinessObjects, as shown below.
There are several valuable documents here, but make sure you grab the Sizing Companion for SAP BusinessObjects BI 4.0.
On a related note, my friends and Diversified Semantic Layer hosts Eric Vallo and Greg Myers are writing the SAP Press title SAP BusinessObjects Administration, scheduled for publication in October 2012.