I can not even recall how many times I was asked by a customer what size the machine should be bought to run our Software.

Unfortunately Tech Support is not even the right address to answer that question, as a purchase decision is closely tied to the answer.
Hence, Tech Support has been limited to the answer: "The biggest machine you can afford" . Many Customers were unhappy with that and have tried to get us to be more precise and that causes a lot of explanation and lengthy discussion. In the end no one is wiser or happier. 

Therefore I am happy to report that at least for OBIEE the decision has just been made a whole lot easier.

Have a look at the note Oracle BI EE 11g Architectural Deployment: Capacity Planning (Doc ID 1323646.1)

The document attached to that note gives you a good overview for teh sizing of the machines that Oracle recommends to run OBIEE (be it a small installation or a bigger distributed installation)

If you have any more questions about this topic and what machines we recommend, then get in contact with  Oracle Consulting or speak to your sales representative.