CPU not supported?

Hi.

Intel does have a knack of obscuring things to this point. So let's get some things out there:

We are currently talking about running things "out of spec". That's just because we don't have enough info. So let's get enough information so that we can make an educated approach.
(So.. TLDR: It will most likely work because P55 chipsets are designed for Lynnfield parts and x3440 is a Lynnfield unit. It's still out of spec and could be considered a "mod".)

The ASUS P7P55 LX has a P55 chipset and according to Intel, this chipset works with Lynnfield products (Which is a codename for a series of chips...). And chipsets are usually designed to support a specific product line, we'll use this to our advantage later on.

According to INTEL's website P55 support goes up to a Core i5-760 ( https://ark.intel.com/products/42690/Intel-P55-Express-Chipset#tab-blade-1-1 )

According to ASUS's website this motherboard supports up to a Core i7-880 ( https://www.asus.com/Motherboards/P7P55_LX/HelpDesk_CPU/ )

So what gives?

When we look at release dates then that's one part of the answer the P55 chipset was released Q3'09 and i7-880 was in Q2'10 ( intel [strike]kindly[/strike] did not update their info on P55, maybe? )

ASUS still says it's compatible, though. Motherboard manufacturers usually should update their CPU compatibility lists according to their own testing when the product itself is not discontinued and active. This is all fine with standard products but we're talking about slapping a server CPU into a consumer oriented motherboard. (Kind of reminds me of people jamming older Xeons into 775 mobos - different ordeal)

MoBo manufactures are most likely bound not to test server grade cpus on consumer grade hardware because of reasons ($$$?).

Usual differences between server and consumer grade CPU's are their grade of goodies within. And yes, Lynnfield xeons are no different. Mainly, they support up to 32Gb of ram and have a broader memory speed support DDR3 800/1066/1333 as well as minor knacks to their favor. (look up the pcie configurations)
The core i7-880 only goes up to 16Gb ram and DDR3 1066/1333

Still, ASUS states the motherboard will ultimately go up to the 16GB limit. (This is most likely bound to the memory controller inside the CPU)

This is where we go forum diving for more insight. At a glance it will work.
Google fu will eventually show up with this, for example: http://www.xtremesystems.org/forums/showthread.php?246609-Does-the-Xeon-3440-work-on-standard-P55-chipsets

So, here is the Products formerly known as Lynnfield list https://ark.intel.com/products/codename/29896/Lynnfield

Happy hunting and good luck with your modification.