memory

I've a relatively small machine (512MB) that I am setting up as a small area
database server.  And I was trying to get the memory balanced out for this
machine.  I don't plan on running anything other than postgresql and whatever
might be required to operate sanely on the network.

So I was changing my shared buffers and found I couldn't really get over 3500
before SHMMAX started complaining.

That being done, I'm running some jobs now on this server and have noticed that
postgres uses only a few percentage points of the available memory according to top.

So, I'm trying to understand why I don't have more memory being used up by these
SQL jobs.  I was assuming that running 100 SQL statements/second would suck up a
lot of memory.

Right now all it seems to burn in CPU cycles more than RAM.

Maybe I don't understand much about how postgres will appear to operate...

But is the memory limited by the shared_buffers * max_connections?