how to update table to make dup values distinct

[PostgreSQL 7.4RC2 on i686-pc-linux-gnu](I know, I know... must upgrade soon)

I have a table mytable like:i |  txt  
---+-------1 | the2 | the3 | rain4 | in5 | mainly6 | spain7 | stays8 | mainly9 | in

I want to update it, adding a ':' to txt so that each txt value is unique.
I don't care which entry gets changed.  I tried:
update mytable set txt=mytable.txt || ':' from mytable t2 where mytable.txt=t2.txt and mytable.i=t2.i;

but this updated both duplicated entries.  

Um, there may sometimes be 3 or 4 duplicates, not just two.  For these, I can add multiple colons, or one each of an
assortmentof characters, say ':+*&^#'.
 

Performance does not matter here.  The real table has 30K rows, ~200 dups.
To clarify, I want to end up with something like:
1 | the2 | the:3 | rain4 | in5 | mainly:6 | spain7 | stays8 | mainly9 | in:

-- George
-- 
"Are the gods not just?"  "Oh no, child.
What would become of us if they were?" (CSL)