finding a maximum or minimum sum

I have a table that looks like so:
userid | amount
---------------  1   |  $500  2   |  $400  2   | $-100  2   |   $10  3   |   $10  3   |   $10

I run a select sum(amount) from payments group by userid
userid | sum
-------------- 1    | $500 2    | $310 3    |  $20

I need to modify this query so it returns the minimum, maximum and 
average sums. Is there any way I can do this?

-Michael
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