Which RAM should I buy to add more to the ones I already have?

Here is a link to the info about my current RAM, Motherboard and CPU.
HWiNFO64 Version 4.20-1960 Central Processor(s) ----------------------------- - Pastebin.com

My main question is what should I be looking for while buying additional RAM so that its compatible with the one I already have? The memory speed and type needs to match right? Anything else?
Corsair - VS2GB1333D4
Corsair - VS2GB1333D3

I already have 2x 2GB of RAM.
My motherboard has 2 more RAM slots free so I was thinking of adding in another 2GB x 2 (4GB) of RAM.

If you can make suggestions on which one I should buy, I'll appreciate it.
My first preference of buying something would be this site.
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Second preference
eBay.in