Database ressources utilisation

Hi!

I am in the design process of a system where I trying to incorporate
PostgreSQL as the framework.

I am trying to figuring out:
- how much memory I need and
- how much disk space I need.

for a given:
- schema (number of tables, views, triggers, contraints...),
- number of rows in all tables
- number of client connecting to the database and their requests
- time spent in triggers
- periods of client requests
- network speeds and overhead
- OS memory / disk space overhead

Does anyone know/guess any formula?

Thanks.

Richard

NOTE: CPU time and network usage can be derived from that.
NOTE2: I know the "explain" command but I don't know how it reach its
result.
NOTE3: I am novice in database system but a bit more familiar in real time
system