I think a market cap of 8-20 billion is appropriate for the current market ($380-$952 BTC).  I think if paypal/ebay/amazon start accepting it, you could make the case for an 80 billion dollar market cap ($3,800 BTC).  If investors begin to embrace the coming ETF and companies like Walmart start accepting it, you could probably make a case for a 200 billion dollar market cap ($9,500 BTC).  If small countries start using it as their national currency and organizations like OPEC use it to price their products, you could see >400 billion dollar market cap (>$19,000 BTC).  I personally think it will continue to grow until it settles in somewhere around an 80 billion dollar market cap for a while.  My $0.02.