W5620 - Fan & Shroud Assembly for Memory Configurations over 4GB or Higher for Dell Computers is the correct part number for your dell system. However, the cost is anywhere from $200 - $400 US dollars.
A work around is to use a kingston HyperX memory fan and change the three pin plug to match the dell server.
Doe anyone know when I can get the proper connector for the dell SC1420 server fan? A picture is shown here.