Outdated docs. The spec sheet is dated 11/2007, the X5470 was released in 2008. So, either there was a BIOS update to add support OR it wasn't listed simply because it didn't exist yet. 1. There are systems with that CPU. 2. They are sold by reputable resellers for that machine. It can only work in a III, and I would recommend updating the BIOS (and other system firmware) to the latest before installing. 
Wondering if someone can clear this up for me. Can the poweredge 2900 III support the xeon x5470 cpu? The confusion around this for me stems from the dell documentation I've found on the poweredge 2900 III which states it supports the xeon 5400 series cpu's BUT up to 3.16GHz. The xeon x5470 is rated at 3.33GHz. However I've seen numerous times poweredge 2900 III's running x5470's. *** a quick search pulled up a few ebay listings for this server with that very cpu. Could there have been a bios update that enabled support for this cpu and the dell documentation is just outdated? So due to the conflicting information I'm seeing before I go and spend the money, can anyone provide me a concrete answer on whether I can run this cpu or not?
Poweredge 2900 III support xeon x5470?