For absolute optimum performance it is best to have the PostgreSQL databases on a drive on their own, so that database access isn't interrupted by any other disk activity. There is a tutorial about setting this up linked from the
Database / PostgreSQL FAQ.
For a system like yours I suspect that the additional setup may not be worth it - if you install PostgreSQL on a drive on its own (the databases will be stored there too by default) then that should be pretty fast. Please bear in mind I don't have direct experience with setting up systems like this so you may get better advice - Kraada will be on later and may have more to say on the matter too.