How To Update If Exists Else Insert In One SQL Statement

In MS Access I can do in one SQL statement a update if exists else a
insert.

Assuming my source staging table is called - SOURCE and my target
table is called - DEST and both of them have the same structure as
follows

Keycolumns
==========
Material
Customer
Year

NonKeyColumns
=============
Sales


In Access I can do a update if the record exists else do a insert in
one update SQL statement as follows:


UPDATE DEST SET DEST.SALES = SOURCE.SALES
from DEST RIGHT OUTER JOIN SOURCE
ON (DEST.MATERIAL = SOURCE.MATERIAL AND
DEST.CUSTOMER = SOURCE.CUSTOMER AND
DEST.YEAR = SOURCE.YEAR)


This query will add a record in SOURCE into DEST if that record does
not exist in DEST else it does a update. This query however does not
work on SQL 2000

Am I missing something please share your views how I can do this in
SQL 2000.

Thanks
Karen