table and index size

Is there a way to determine the size of a table? an index?
I didn't see anything in the reference manual about this so just did a
'du' before and after creating and populating a table and then again after
creating an index on the table.  The numbers I got were shockingly low.

I created a table with two int4 columns and inserted 100 K rows.  The
change in disk usage was only 4.3 KBytes, or .17 bits per integer.

One of the columns had values ranging from 0 to 3.  The other had random
32 bit integers.  Making an index (btree) on either column used an
additional 1.8 KB, or .14 bits per row.

These numbers were a little surprising, so I du'ed / to see if the data
was being hidden somewhere, and got the same differences.

So, I guess there are two questions here.

1) Do these numbers seem reasonable to someone with a little more
Postgresql experience?

2) Are there commands to determine directly the size of a database, a
table and an index?

One further question, if anyone can comment.  I have the New Riders
PostgreSQL Essential Reference book.  It claims that in my database
directory I should find some files with plain text names.  As an example
they say that "$PGBASE/payroll_idx" is the name of the payroll index file.
All of the files in my database directory have opaque names like
"$PGBASE/16555/16475".  Is there a way to generate more human filenames or
is the New Riders book just off base on this?

Thanks,
 Damon