Help with SQL staterment

Hello list,

Using PostgreSQL 9.0.2 I'm trying to write a sql statement to extract specific rows from the following data set. I've looked at the documentation that talks about the Max() function but the example doesn't go far enough:

The sample fields/dataset are as follows

PaymentKey, ContactKey, PaymentDate
1, 100, 01/01/2011
2, 100, 12/30/2010
3, 100, 12/31/2010
4, 101, 01/02/2011
5, 101, 12/25/2010

What I want returned are rows grouped by contactkey showing the corresponding payment key and payment date for the record that has the max (newest) payment date. So I would want my result set to look like this:

1, 100, 01/01/2011
4, 101, 01/02/2011

I would be using this query as a sub query that is (left) joined to a master query by ContactKey

Any assistance would be greatly appreciated.

Thank you,

Chris