My name is Arnold and I'm pretty sure it does mean eagle power or something similar because I remember getting a book mark with my name and meaning on it when I was like 10 or 12 (I'm 32 now) and it said "Strong as an Eagle".
Site says from German, 'eagle' -- RIGHT. Plus 'wald' for power. -- WRONG. Wald in German means forest or woods. Arnwald means eagle woods. There are places in Germany and Austria with exactly this name.
In German : Der Adler waltet oder verwaltet.