update field with ranking

Hi, is there an efficient way to update a table with the rank of that row, given a specific order by

lets say i have

Table: users
userid int
score int
rank int

select * from users order by score DESC

I want to update the rank field, to the corresponding row number in the above query

Thanks

Reading this article may help :

http://arjen-lentz.livejournal.com/55083.html

Interesting and useful link, thanks!