Index on wide column

Hi all,

I need to perform a tree traversal on a big table (millions of rows).
To avoid recursive queries, one for each non-leaf node, this table has,
in addition to its 70 columns, a VARCHAR(30000) column that is used
exclusively to sort the rows with the required order. The actual content
length in that column is expected to be, on average, much less than the
declared limit and the text will be composed of digits and letters only.

Please, are there any restrictions about using such a wide column to
order a table? Can an index on that column help?

I'm running PostgreSQL 7.1.2, on Linux 2.2.16, compiled with options:
--prefix=/usr/local/pgsql --enable-locale --enable-multibyte

Regards,

Antonio Sergio