Monish,
Have you checked details within the BI4 Sizing Guide
Sizing and Deploying SAP BI 4 and SAP Lumira
TechED Presentation
SAP BusinessObjects BI4 Sizing – What You Need to Know | SAP TechEd Online
Sizing Estimator:
http://www.sdn.sap.com/irj/scn/index?rid=/library/uuid/1055c550-ce45-2f10-22ad-a6050fff97f1
Regards,
Ajay