According to this website [0], the above-ground gold is valued at about $8.5 trillion USD. As this website [1] points out, the M2 amount of USD in the world's supply is $9 trillion. USD makes up about 15% of the total world's supple of $60 trillion.

This HN thread is by far the most interesting one I've read about speculating Bitcoin's future value.

[0] http://www.numbersleuth.org/worlds-gold/ [1] http://erikaugust.com/blog/thought-on-bitcoin