I don't think that it is logical to assume that the Dow, Nasdaq, Gold or Silver will double in price in the next two years.  

I do think it is almost gauranteed that one of them will be up 10% while another will be down 10%.

If that is a valid assumption Bitcoin will have to increase in value 10x to ~$110 for memorydealer to win his bet.

I would consider buying 100 Bitcoins for $1100 as a hedge and making this bet as pretty much a risk free wager.