More RAM will not necessarily speed up renders and as long as we don't have a frame of reference by knowing the specs of your current system and settings, we can't know anything. That aside, you could throw on a computationally heavy effect on your clips and they will not render any faster. So what is it to be? You need to be much more specific and precise.
A few things influence smooth playback: the comp's frame rate & resolution are at the top of the list. Processor speed is another.
So I'll go worst-case and say that you need 96 GB of memory. Fair enough?
Sorry I didn't know you needed that. My machine is Processor speed of 2.0, 2TB of space, 8Gb of RAM (as of now). It is upgradable to 16GB of RAM though. Is that what you need?
My processor speed is 2.0 (I know kind of low), but I was hoping that I could get away with only buying more RAM and not upgrading my processor. I have 2TB of space and 8GB of RAM as of now.
You won't be able to do 4-minute-long RAM Previews even if you trick your machine out to 16 Gigs.
You probably won't get realtime Ram Preview playback if you're making 4K comps at 59.94 fps.
You may want to look at the RAM Preview tricks mentioned in the View and Previews section of AE Help... and ignore Adobe's advice to use the play button in the preview panel or to hit the space bar -- neither is a proper RAM Preview.\
This issue of realtime RAM Previewing has been going on for decades. Adobe created ways to help people cope, like reducing comp viewer size to to 50%, comp viewer resolution to half, defining a region of interest... the list goes on.
Processor speed makes a huge difference in how fast AE renders (in general). It's the biggest contributing factor, for the most part.