Sizing hardware is always a challenge. The issue here is that production sized hardware usually needs to be procured even before the project has started. This is usually the case with organizations where Oracle and Oracle EBS is being installed for the first time. There is no prior version of the application, to benchmark against.
Most of the hardware vendors provide sizing spreadsheets that can be filled out, and a rough estimate of the computing power needed can be reached. This exercise usually helps in clarifying the SAN and disaster recovery architecture, but does not get to the specifics of the production hardware.
Each organization has unique access patterns and transaction patterns, and hence it is genuinely difficult to extrapolate hardware sizing between different organizations. The best approach is to procure the production database server (since this can usually be sized with a fair degree of accuracy) or the post-production development server. These can be used to start the implementation process, and once a certain level of organization specific configuration has bene completed, then production benchmarking can be accomplished.
There is some assistance available in the for of white papers and benchmarking for typical transaction loads. This link has a list of white papers on sizing for IBM, HP and Sun machines. This link provides detailed performance information for representative transaction loads.
When it comes to estimating the network bandwidth requirements the information is very scattered. There are no bench marks and thumb rules need to be applied. A usual CRM/imodule would have the same bandwidth requirements as a normal internet user. HOwever, power users, especially in the demand planning and forecasting space will require a higher bandwidth