I'll chip in a bit; some other folks could provide even better answers.
Considering the RAM usage
Graphics generally impact the video RAM (the VRAM), not the system RAM as you seem to mention. Often, computers will have much more system RAM than video RAM. Also, VRAM will often be occupied more by textures than by actual vertices that compose the mesh. If you have a low-poly mesh and a hi-res texture, you could bust your (V)RAM.
will it be better to use low - poly terrain
We can't answer that.
What kind of game do you want to make? Would it make sense to have a low-poly terrain and a hi-poly main character(s)? Depending on the game, you can "cheat" and load dynamically only the terrain that is visible to the camera and what's immediately surrounding it; you unload what's not visible, freeing up the memory for what matters.
Keep in mind that using low-poly terrain can be an advantage w.r.t. physics. The less polygons there are, the simpler the collision checking (where a completely flat ground is the simplest).
It's a balancing act between all your needs.