This highly depends on exactly what you're doing.
If you have a AMD APU processor like the 7850k you will benefit greatly from using faster ram you'll notice higher FPS rates in games. if you want to do more multitasking while playing games such as browsing the web when you get stuck on a level or mission you'll want to get more ram and faster ram. If you're using a dedicated video card, you might as well get a better processor, intel I5s are perfect for most people i7s are great but they cost more.
Another way to increase speed performance is by upgrading to a SSD (solid state drive) the Samsung 850Evo series has become the new standard in commercial SSDs, traditional HDDs are limited because they use mechanical moving parts and have much longer response times. if you're willing to sacrifice storage space for speed this is the single best upgrade you can do for your home computer. You will especially enjoy much faster speeds for every day tasks like browsing the internet, loading video files and games etc.
CONCLUSION: get more RAM and faster RAM at least two 4gb sticks equaling 8gb capacity but of course 2 8gb sticks 16gb will let you do more but they will also cost more 2133 to 2400mhz memory is best, 1600mhz is for typical everyday nothing special home computers; use a SSD as your boot drive and for every day tasks like gaming and surfing the web, then use a separate cheap high capacity HDD for mass storage of files for video and audio. You shouldn't need anything bigger than 1tb capacity for a hard drive and a 250gb SSD as a boot drive. that's the sweet spot for today's gaming home theater PC set ups.