Performance and Clustering

hi,
Today is my first day looking at PostgreSQL
I am looking to migrate a MS SQL DB to PostgreSQL :) :)
My customer requires that DBMS shall support 4000 simultaneous requests
Also the system to be deploy maybe a cluster, with 12 microprocessors

From what I have read, PostgreSQL has really good performance and reliability but I would like to get some numbers, not sure if somewhere in the wiki some of this data is available.

I am currently in a research/explore face on the project.
I am looking at PostgreSQL and MySQL.
In future this will be a showcase success story, and we plan to public it once we complete the project.
I will be really happy if you can point me in the right direction so I can get strong data to make me choose PostgreSQL :)

Thanks,
--
Ing. Jaime Rodríguez Quesada, Mag.
Liberux S.A.
http://www.liberux.com