So sorry... did not realize your limited needs, but my post was actually also for future forum members wanting the latest on how to upgrade the xw8400.
The answer to your specific question is no. To use the 5400 series you need to move up to the xw6600 or xw8600 generation of workstations, and the latest Quickspecs for those will tell you the 5400 series processors you can use.
An example, but for the xw8400:
Google "xw8400 Quickspecs" and the xw8400-approved processors are listed on page 3:
