Are you asking about system ram or graphics vram?
1. On system ram, once you have enough for the game, more does not buy you anything.
At one time, with 32 bit architecture, no game could use more than 2gb or possibly 3gb.
There are still many games built to that limitation. Then with 64 bit architecture the ability to use more than 4gb was possible.
8gb was considered as more than enough.
Today, I think 16gb is the safe amount to get.
There is no downside except for cost to buying more than you need.
2. On vram a similar concept applies.
If you have enough, you have enough.
VRAM has become a marketing issue.
My understanding is that vram is more of a performance issue than a functional issue.
A game needs to have most of the data in vram that it uses most of the time.
Somewhat like real ram.
If a game needs something not in vram, it needs to get it across the pcie boundary
hopefully from real ram and hopefully not from a hard drive.
It is not informative to know to what level the available vram is filled.
Possibly much of what is there is not needed.
What is not known is the rate of vram exchange.
Vram is managed by the Graphics card driver, and by the game. There may be differences in effectiveness between amd and nvidia cards.
And differences between games.
Here is an older performance test comparing 2gb with 4gb vram.
http://www.pugetsystems.com/labs/articles/Video-Card-Performance-2GB-vs-4GB-Memory-154/
Spoiler... not a significant difference.
A more current set of tests shows the same results:
http://www.techspot.com/review/1114-vram-comparison-test/page5.html
And... no game maker wants to limit their market by
requiring huge amounts of vram. The vram you see will be appropriate to the particular card.
I might add that if you are modding a game, you may not be using vram as well as the game developers.
In that case, more might be better.