SNPP223CC/2G is the Dell Specific Part number for ram that WILL WORK.

Dell computers are fussy about ram. See how this recommended Crucial memory compares with the ram you have--

http://www.crucial.com/usa/en/optiplex-780-desktop/CT2367680?gclid=CjwKCAjwhevaBRApEiwA7aT533JSE4HDQT3Cuugst0JVST3CY7NcRFo89XgnxruasqaglBIdNQVWkBoCQoUQAvD_BwE&ef_id=W1cmwAAADnF4M5J_:20180727181935:s 

Always best to match the memory chips and not mix ram from diff suppliers.

I checked and i found out that my ram is 1,5 v and crucial is 1,35 but i dont think it shoud matter that much, or should it ?

Crucial has both 1.35v and 1.5v versions of the ram.

SNPP382HC/4G

https://www.newegg.com/Product/Product.aspx?Item=N82E16820675015

Ram that supports either voltage is common.

The issue is LOW Density INTEL compatible Ram.

https://www.datamemorysystems.com/parts/dell/SNPP382HC%2F4G/

http://www.memorystore.com/product.asp?oemnum=SNPP382HC%2F4G&fid=842

https://www.amazon.com/Dell-Certified-Optiplex-SNPP223CC-A4051419/dp/B005CPUF12/

https://www.amazon.com/2X4Gb-Memory-Compatible-Optiplex-Desktops/dp/B00AAL6KHG/

https://www.amazon.com/Dell-memory-DIMM-240-pin-DDR3/dp/B0060R9P44/

So i have to buy low density ram ? correct?

Because i do not live in US i have to buy it from local HW store. So i go in there and say that i want low density intel compatible ram, right?

Thank you!