Strictly speaking, there is no such thing as a "plpgsql script" - PL/pgSQL is the default procedural language of PostgreSQL. It's either an SQL script or a plpgsql function / procedure. Your example seems to indicate an SQL script.
You could create a (server-side ) plpgsql (or sql) function instead, that takes any number of arguments. It's very simple as long as the arguments are
values. It gets a bit more complicated if the arguments include identifiers. Then you'll have to use PL/pgSQL with dynamic SQL and
PL/pgSQL is pre-installed by default in PostgreSQL 9.0 or later. You have to install it once per database in Postgres 8.3, though:
CREATE LANGUGAGE plpgsql;
Speaking of the version: you should consider upgrading to a current version of PostgreSQL. v8.3 is very old by now - updates will be discontinued (end-of-life) early in 2013.
Since you seem to have a ready SQL script I'll demonstrate an SQL function. Simple dummy function with two integer arguments:
CREATE OR REPLACE FUNCTION func(int, int)
LANGUAGE sql RETURNS void AS
UPDATE mytbl1 SET col1 = $1 WHERE id = $2;
UPDATE mytbl2 SET col1 = $1 WHERE id = $2;
You can find many more sophisticated examples for plpgsql here on dba.SE or on stackoverflow.
You can call this function and hand in parameters in a shell script:
Basic example for a call in a shell script that uses input parameters for integer parameters (no single-quotes around the value needed):
psql mydb -c "SELECT func($1, $2)"
Or with any data type:
psql mydb -c "SELECT func2('$1'::text, '$2'::numeric)"
-c executes one command string and then exits.
More about command line arguments of psql in the manual.