I had to find this the hardway, you need the BIOS 2008.10.27
My original motherboard wouldn't boot with my 2x Xeon 5130 (NO POST SCREEN)
Purchased another from eBay under 40USD and you can buy it with 1x old CPU already on it.
Needless to say I removed it and installed the other ones.
Booted with the old CPU that came with it and flashed it with 2008.10.27
Once done I turned it off, installed 2x Xeon 5130 and it WORKED!!!!
Yeah... it really shocked me that it worked while HP was offering On-Site Tech with contract (expensive!)
Seriously... some time I wonder if HP cares that we are trying to re-use good things.
I should stick with Dell's because their system take any hardware like a real PC should.
Hope this helps out and updated the BIOS to 2008.10.27