Is it a gold-plated nickel or a $5 gold half-eagle? One is worth 5 cents and the other a few hundred dollars. If it has an eagle on the back and a D mint mark it is a gold half eagle. The confusion is because both coins are almost exactly the same size. However the half-eagle will be about 60% heavier than a nickel.
At current (08/2008) gold prices a 1907-D half eagle is worth $220 to $280 depending on how worn it is.