Ya don't be this guy:
See what CPU-Z tells ya first. Then go look up the mobo details and find the max RAM allotment.
If the mobo can't handle 32GB by chance, then you might as well just stick with that single 16GB stick, or buy two new 8GB DDR4 sticks. You can always sell that 16GB stick if you need the money, and maybe those 8GB DDR3 sticks as well.
If you're going to buy new RAM, faster (higher MHz) is better in general, more useful for Ryzen though, but you also would need to know what the mobo and CPU support for max RAM speeds. If you buy faster RAM then your system supports, it should still work, but it will be down clocked and you will have paid for faster RAM for nothing. If your mobo doesn't support 32GB, then while your searching, look up the max supported RAM speed. CPU tab info will also give you what you need to search it's max supported RAM speeds. Whichever RAM speed is the slowest max between the CPU and mobo, that's the max you should bother going with. The faster the RAM, the more it costs as well, so depends on what your budget would be.