I think more RAM improves your FPS when running at higher resolutions. Correct me if I'm wrong.
In modern games going from 2gb to 4gb will give a fps boost of maybe 25%-30% i would guess.
Going from 4gb to 8gb will net you like 1fps and a little less texture popup on games like GTA IV.
Going from 8gb to 16gb gets you zero fps increase and eliminates texture popup in GTA IV etc.
If OP is talking only about ram speed with say £50 sticks compared to £100 sticks with better timings then about 1-2fps at most even with extreme timings.