I have an I74790K + 980Ti combo currently with 8GB (2x4) of 1600Mhz Corsair memory. A friend gave me 8GB (2x4) of generic 1333Mhz memory from his old PC. Should I just use it or is it worth it to try and buy matching 1600Mhz memory?
Not a whole lot of difference between 1333 Mhz. and 1600 Mhz. You could use both by trying to set the XMP for 1600, but the BIOS will default to 1333 if you don't. You won't see much difference in having 16 GB of RAM either unless you are editing or having a boatload of windows open on the net or streaming while you are gaming, which most people don't do anyway.
The biggest and best jump is from 1333 to 1866.