Concatenating several rows with a semicolon

Hello,

I'm working on a small app, which receives a list of 20 players in XML format.

The initial version works ok and I use there just 1 SQL statement and thus
it is easy for me to fetch results row by row and print XML at the same time:

select u.id,
u.first_name,
u.city,
u.avatar,
m.money,
u.login > u.logout as online
from pref_users u, pref_money m where

m.yw=to_char(current_timestamp, 'YYYY-IW') and
u.id=m.id
order by m.money desc
limit 20 offset ?

My problem is however, that I need to add more data for each user
representing their statistics over the last 20 weeks.
And that data is in separate tables: pref_money, pref_pass, pref_game:

# select yw, money
from pref_money where id='OK122471020773'
order by yw desc limit 20;
yw | money
---------+-------
2010-52 | 760
2010-51 | 3848
2010-50 | 4238
2010-49 | 2494
2010-48 | 936
2010-47 | 3453
2010-46 | 3923
2010-45 | 1110
2010-44 | 185
(9 rows)

For example for the table above I'd like to concatenate
those rows and add them as an XML attribute for that user:

<user id="OK122471020773" first_name="..." city="..." ...
pref_money="2010-52:760;2010-51:3848;2010-50:4238;...." />

so that I can take that attribute in my app and use it in a chart.

My problem is that I don't know how to bring this together
in 1 SQL statement (i.e. the SQL statement at the top and
then the concatenated 20 rows from 3 tables).

Is it possible? Maybe I need to write a PgPlSQL
procedure for each of the 3 tables and then add them
to the SQL statement above? But how do I concatenate
the rows, should I create a PgPlSQL variable and always
append values to it in a loop or is there a better way?

Thank you for any hints
Alex