T1700 PRECISION memory upgrade

You will need DDR3L-12800, 1.35V, UDIMM, not RDIMM.  

Thanks @Chino de Oro
So, I see PC3L-12800, PC3L-12800U, PC3L-12800E...
Which are ECC unbuffered​ UDIMM ?

PC3L-12800U, ECC unbuffered.

@Chino de Oro​ , so I am only able to upgrade from 16GB (4x4GB PC3L-12800U) to 32GB using 4x8GB PC3L-12800U ?

For example this one ? https://www.crucial.com/memory/ddr3/ct102464bd160b

@OLIWOOD​ , yes, for a 32 GB configuration, you will need 4 x 8 GB of PC3L-12800 UDIMM.  Crucial is a good place to purchase.  They provide guarantee to work, and accepting return.

Review your needs, 16 GB is sufficient for every day usages.  Only upgrade to resolve memory issue or because you can source them at dirt cheap price.

Last question @Chino de Oro
Can I mix 2x8GB PC3L-12800U with 2X4GB PC3L-12800U ?
24GB is enough for me. 
​ 

Then, upgrading memory will be a must.  Just note that your Crucial link was not a compatible memory for your system.  Mainly, look for PC3L-12800 UDIMM.  Your system can take both ECC and non-ECC.  However, with ECC memory, you can upgrade to maximum of 32 GB.  For non-ECC, you can only upgrade to a maximum of 16 GB.  Mixing capacity is not support but it could work.

You can review this YT video for memory upgrade.

@Chino de Oro​ the crucial is DDR3L-12800 UDIMM https://www.crucial.com/memory/ddr3/ct102464bd160b

Am I wrong ?

@Chino de Oro​ and everybody,
I have read tons of posts about upgrading the T1700 memory and the more I read the more I am lost, I have never seen something like that, so crazy.
You told me 4x8GB PC3L-12800U ECC Unbuffered.
I just seen the YT video and the guy is installing 4X8GB PC3-14900E ECC Unbuffered.
So U or E ? PC3L or PC3 ?
I just don't know what to buy ;-)

Compatible memory specs were given many times in previous comments.  Since there are different type of compatible memory for your system, stay with the same specs of your current existing RAM, just with higher capacity.  Purchase from seller whom provide warranty and accept return.  YT video was suggested for safety upgrade process.  Crucial is a good seller but your provided link yielding non-compatible for T1700.  See screengrab below.

To upgrade from 16GB to 32GB, you will need 4 x 8GB modules of ECC (12800E).

Here is a specific module that should be compatible and work with your system.  Scroll down for detailed description of specs information.  

The information I provided was/is the same as from everyone else.  I think the issue at hand is the lack of understanding and therefore, the same question being asked over and over again.  

For instant, people say Dell specs are wrong.  That non-ECC memory capacity can be upgraded to 32 GB.  The specifications are the validation of components Dell has tested, will provide support for, and guarantee to work.  While there are some limitation by technology, others components were just limited to within the OEM stocks which Dell would carried with system configurations.  On several Dell models, users have succeeded with upgrading to higher capacity on various components such as memory and storage capacity.  Just because unsupported configurations could work, it does not mean that Dell specs were wrong.

Registered RDIMM would not work with your system, and the Xeon processor can support ECC, hence the suggestion for full capacity upgrade was ECC unbuffered (UDIMM), 1.35v memory.  Note that the E in 12800E stands for ECC.  The U in 12800U is for Unbuffered.  The L in PC3L indicates 1.35v in this case.  So, don't be at a lost thinking that they are different thing.  Sometimes, there might be issue with a purchase, such as DOA, wrong item.  Be responsible and don't fault me for trying to help.  Take precaution while working on system board.

Thank you so much @Chino de Oro​ for your patience en precision !

@bradthetechnut​ the problem is that most of the conversations I have read are not solved.
This one is SOLVED, thanks community :-)