I went from 4GB to 12GB without gaining an extra frame or less stutter.
I even tried the ramdrive thingy but didn't do nothing to me.
Mind you, I got a clean OS without any other stuff running in the background except what I need for CloD.
If you're photoshoping, editing some music and write an essay while flying clod then perhaps more RAM would be good.
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Edit: One other aspect is how much VRAM you got. I can imagine that with 512MB on the g-card you could use more RAM.
Good luck.