What RAM should I add to my Win10 System?

Hello dear fellow members,
I want to speed up my Windows 10 computer.
I have little clue about what to pay attention to if I buy extra RAM.
What RAM should I add to my system? And in what slot should I place them?

I now have:
2x 2GB DDR3 240-pin DIMM at 1333 MHz
( brand: Adata Premier Series PC3-10600 )
I have a Asus P7P55D LE motherboard with 4 slots (2 blue slots currently used)

I want to buy 2 more 2GB DDR3 so i will have 8GB RAM in total.

Thanks in advance!

Best bet it to get two stick of this:
CORSAIR 4GB 240-Pin DDR3 SDRAM DDR3 1333 Desktop Memory Model CMV4GX3M1A1333C9 - Newegg.com

And you can use this to find memory.
System Build - PCPartPicker

You actually need to read the Mother Board Manual to find out exactly what brand of Memrory that your Mother board supports.
It is better to get the same brand as what you currently have and put in the remaining slots.

In addition, Mixing different brand is not always a good option even though they have the same spec and that's why manufacturers always sell them in pair.

@Foreign Frisian

If it were me I would install a set of compatible new 2x4GB RAM in the blue slots and then sell the old RAM on ebay

No it is not a definitive list. It just means that those listed have been tested to work. They won't have tested all models though as there are just way too many.

Another good way to select compatible modules is to go to a reputable memory manufacturer site. Personally, I buy mostly from (in alphabetical order) Corsair, Crucial, GSkil and Kingston.

All of those manufacturers also have their own lists of their RAM modules they have tested with specific motherboards. So you just check their listing against your Asus P7P55D LE on their database. :)

Some RAM manufacturer databases to start you off:
http://www.corsair.com/en-gb/memory-finder
http://uk.crucial.com/gbr/en/advisor
http://www.gskill.com/en/configurator
http://www.kingston.com/us/memory/se...pe=Motherboard

The new and old RAM modules will almost definitely work together but there is also a chance it could impact overall performance. I always recommend using only one model of RAM to ensure best performance

And yes, have one set in blue and one set in black. Each color denotes a separate channel and you want each memory type occupying its own channel. Please refer to your motherboard manual for confirmation as to how this works on your motherboard model.

Edit: Added the line in bold above. It is only in bold to mark it as an edit and does not denote any extra importance :)