select for update question

Hello there.
I'm looking into how to update a row in a table while protecting it
from access by others.

so far I've come up with this solution:

create function dostuff() returns void as $$
declare
  tmp integer;
begin
  select id into tmp from tableX where id>305 limit 1 for update;
  update tableX set some_field = some_value where id=tmp;
end; $$ language plpgsql;

will that guarantee that  the row I selected wil be updated within
this function and no one else can sneak in between and update or
delete the row?


What would I use if I would write

lock table   tableX  IN .... MODE  at the start of my function?

Any particular benefit with either method?