size cost for null fields

I have a table with lots and lots of rows (into the millions), and I want to add some information to it.  The new data
consistsof a VARCHAR and a BYTEA, and it will almost always be null -- let's say only one row in 10,000 will have
non-nullvalues.  I'm trying to decide whether to add the new data as columns in the existing table, or a side table
linkedby FK.
 

Looking at the Database Page Layout page <http://www.postgresql.org/docs/8.1/static/storage-page-layout.html>, it seems
tome that I'll pay the minimum storage cost for these two fields (4 bytes for the VARCHAR, 4 bytes for the BYTEA) in
everyrow, regardless of whether they're null.  When the fields are null, there'll be bits set for them in the null
mask,but the alignment and size of the row won't change -- everything else will be placed as if there were zero-length
valuesin the two fields.
 

Do I have this right?  If so, the side table sounds like the right choice....

    Vance