Faster count(*)?

Salve.

I understand from various web searches and so on that PostgreSQL's MVCC
mechanism makes it very hard to use indices or table metadata to optimise
count(*).  Is there a better way to guess the "approximate size" of a table?

I'm trying to write a trigger that fires on insert and performs some
maintenance (collapsing overlapping boxes into a single large box,
specifically) as the table grows.  My initial attempt involved count(*) and,
as the number of pages in the table grew, that trigger bogged down the
database.

Any thoughts?

-O