Can I mix RAM (same type) of different capacity? (2x4gb and 1x 8gb)
I have an ASROCK 980DE3/U3S3 R2.0 MOBO, and currently 2X4gb Ram. I want to know if I can buy one stick of 8gb RAM of the same RAM (PAtriot Signature ddr3 1333) and configure it as 2x4gb in the first two slots, and the 8gb piece in the 3rd slot? Or do I need to get another 2x4gb set or something?
Technically, you can mix RAM as you describe. However, there is one issue that's unavoidable and one that's possible.
The unavoidable issue is that the new memory will be slower. One stick, single channel. Some motherboards will run your first 4 GB in dual-channel and only the last 8 GB in single-channel. Some will run the whole kit in single-channel. I don't know which your board will do. It's best to buy TWO 8GB sticks. Note that some boards used to be able to make dual-channel out of two 4 GB sticks on one channel and one 8 GB stick on the other, but I have no idea if this "feature" is still around.
The possible issue is that mixing modules of even the same capacity of the same make and model has been known to lead to a failure to boot or other issues. RAM these days is less plug-and-play than it was when I was younger; there's a reason it's sold in kits of pairs or four of a kind, because they have been tested together. Sometimes, people mixing "identical" RAM will find that the motherboard won't start, or stay stable, unless they raise the memory voltage and / or increase the latency.
So the most common advice you will get is to buy a kit of two 8GB sticks and use only those, ignoring the existing 4 GB sticks. At the high speeds and tight tolerances of today's memory, it's best to use matched sets and avoid the risks of mixing. That said, some people mix happily. I've done it myself.
They might play they might not, see my article (item 2) on mixing DRAM here:
also above, WyomingKnott is talking about Flex mode, more on that (item 7) in this article: