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.

Thanks a lot @Chino de Oro​

I run out of memory everyday, I use to have a lot of softwares open, lot of tabs in my Chrome, etc... this is the reason why ;-)

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.

@Chino de Oro​ I have read a lot in that community and others and I still don't know ?
Some people say that the DELL specs are wrong ! That you can have 32GB with Non-ECC ?
Some people say PC3L 1.35v other PC3 1.5v...

Some peolpe say 12800U other 12800E...
So again community, I need PRECISION ;-)
I have a DELL PRECISION T1700 with a XEON E3-1245 and 4x4GB PC3L-12800U.
I want to upgrade my memory from 16GB to 32GB, what kind of memory ?
4x8GB PC3L-12800U ? PC3L-12800E ? PC3-12800U ? PC3-12800E ?
Thanks in advance :-)

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.  

A-Tech RAM (which I think A-Tech slapped their label on Hynix RAM) is what I've used for years.  Good way to save $ on an older machine.  It also has a lifetime warranty.  It's what Chino linked above.

Try not to worry about what everybody else is saying and just pay attention to what Chino is saying.  Chino is going to say what works, not necessarily what everybody else is saying.