Is the value of a 1904 20 dollar gold piece at least 600 dollars?

The value of any coin is at least the "melt value," which would be the value of the component metals. As of December, 2006, the value of one ounce of gold is well over $600. The $20 gold piece in the US has always contained one ounce of 24K gold; the coin itself weighs more, as the ounce of gold is alloyed with other metals.