Regarding to what extent Galadriel's power is due to her possessing Nenya, and to what extent it is Galadriel herself...
When Galadriel threw down the walls of Dol Guldor, recall that this was after the One Ring had been destroyed. Since the Three Rings lost their power at that point, Nenya was now just a pretty piece of jewelry. So however Galadriel threw down the walls (spells or building up her biceps :-), it was her herself.
Of course having Nenya didn't hurt. I think it was Galadriel who said to Frodo that the Ring gave power according to the stature of the bearer. Although this statement referred to the One Ring, it may apply to the others as well. In short, Galadriel was able to be so powerful with her ring, because she was already so powerful on her own.
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