You have a pretty decent system, I'd say an nvme m.2 ssd or better graphics card.
Well it depends, what games are you looking to play? I wouldn't be able to give you good advice unless I know what you plan on playing/doing with your system Otherwise you would most likely be over/under-spending
Also most modern motherboards have multiple SATA ports, and usually only one gets sacrificed when you plug in a nvme m.2 module. Should leave you with at least 3 free, unless you have some kind of odd or low end motherboard. Looks like your motherboard has a total of 6 SATA ports.
It's always good to have more storage, you can install least used programs on old spinner drives along with data like movies, music etc.
I recommend sticking with good brands such as Samsung, Western Digital, and even Adata if you want to save a few bucks (I've had a good experience with all 3)
You'll want to reinstall your operating system to the nvme drive to really see the benefit, I recommend 256GB, 512GB or 1TB depending on your budget. 128GB is kind of small for OS and programs to live on.
To be honest your GTX 1070 is a pretty powerful card, I would wait until nVidia's next generation instead of buying an RTX card. This way you skip the generation of cards that really didn't perform like everyone thought and maybe they'll get ray tracing performance usable by the next generation. Also gives AMD time to respond with their line of cards as well. I think you'd be better served waiting a bit.