We have seen many (most) failures with the Kingston Value Ram (KVR).  This is not just with a Dell, I've seen it on other computer forums.  I would not use KVR Ram.

The Crucial memory listing shows max memory as 8GB.  Crucial is what we recommend for non-Dell branded memory as it is guaranteed to work in a Dell.  Dell's, and especially PC's in that era, have a history of compatibility problems with non-Dell memory. 

https://www.crucial.com/usa/en/compatible-upgrade-for/Dell/optiplex-755-series-%28desktop-mini-tower-and-small-form-factor%29

KVR800D2N5K2/4G is HIGH Density Ram.  Max Dimm size for this model is LOW Density INTEL compatible Ram 2 gig modules.

https://www.kingston.com/datasheets/KVR800D2N5K2_4G.pdf

Max Ram for this model is 8 GIGS not 16.

This is not EVER going to work.

Specs: DDR2 PC2-5300 • CL=5 • Unbuffered • NON-ECC • DDR2-667 • 1.8V • 256Meg x 64 •

https://www.crucial.com/usa/en/optiplex-755-series-%28desktop-mini-tower-and-small-form-factor%29/CT722584

Thank u for reply

So what kind Of Ram Matches this computer ? i see some people using a 12gb ram In dell755

ok thank u very much

Optiplex 755 Tech Guide

Max RAM is still 8GB.  Not only was it stated by speedstep and fireberd (Rockstars) the max is 8GB, but it's also in the tech guide.

Is this in other forums you see people using 12GB in their 755?  No links or references were given.

Referring to the very first thread in this forum, it's not stated what size your 755 is - MT, DT, SFF, or USFF?  If you had the USFF, the max total RAM is 4GB.

To get the max out of my 755 MT:  CPU = C2Q Q9650; RAM = 8GB; OS is on a Crucial SSD.

If you're not sure how to find RAM that matches your PC, "Optiplex 755 RAM" is an example of a search term.  Crucial's site has you enter the PC model.  Depending on your budget and considering the 755 is 11-12 yrs. old, A-Tech RAM is on eBay and Walmart.com.  I use A-Tech and like Crucial, it has a lifetime warranty.

Tip:  If you're not sure if you found the right RAM, Crucial can be emailed through their website and A-Tech emailed through eBay.

You're welcome.  I also Googled and ran across this thread.  But Dell never tested the 755 with higher than 8GB RAM.  That's why higher than 8GB is unsupported and companies, like Crucial, that make RAM, will tell you the highest is 8GB.

If you have Windows, System Info in the Control Panel will tell you total use of the RAM.  Something like 0.5GB will be set aside for the OS.

As long as you have 64-bit Windows and not 32-bit, you should see all of your RAM.  32-bit Windows only utilizes 4GB of RAM.  It's not stated what OS you have.

I went back thru the last 15 years of posts.

There was SPECIFIC vendor that made 4 gig dimms that worked in some models. Patriot  psd24g8002 

HOWEVER the cost of these modules is $70 TO $200 each so 4 of them would be $280 TO $800.

Obviously scalping and no way worth that kind of money.

https://www.memory4less.com/patriot-4gb-ddr2-pc6400-psd24g8002

https://www.amazon.com/Patriot-Signature-DDR2-800MHz-PSD24G8002/dp/B003X1QAJ8

HIGH DENSITY MODULES never work.

Must be LOW density non ecc non registered INTEL compatible ram in order to work.