It may or may not work, but it PROBABLY won't.  With the latest BIOS, 5400-family CPU's are supported, BUT the specs outline which are officially supported by the BIOS code: Up to 3.16GHz (X5492 runs at 3.4GHz)1033 or 1333 MHz FSB (X5492 is 1600MHz)2x6MB (12MB) cache (X5492 meets this requirement) The X5492 exceeds the supported specs (except for cache) and any of them may keep the system from working with it.  However, there is a chance that it will work too.  If you try it, be sure to tell us whether it worked or not, so as to expand our collective knowledge of what the current specs are. [URL]  
Hoping someone can confirm if a Dell PowerEdge 2900 III supports an Intel Xeon x5492 3.4Ghz.  Latest Bios 2.7 shows support for 5400 series.  Any information is appreciated.
Dell PowerEdge 2900 III - CPU Upgrade Xeon x5492 3.4 Ghz