Am I correct in assuming that a Pentium D 915 Presler has the same architecture of a Pentium D 930 Presler and therefore is just as compatible with OS x86?

Yes.

i am new to this and need clarification.

On the Pentium D 915 - it has dual core (2 cpus on one core)

Does OSX actually use both CPUs or does only one core get used
and the other just sit idle?

Also if I upgrade from a Pentium 4 550 CPU to a Pentium D 915 - will the system optimize automatically or do I have to do anything?

Thanks