Read through all of this and here is the solution to fast transactions without the fear of loss due to double spends. Simply accept any transaction with the value below REWARD * Gold value/2. (NOTE: I hate valuing items with no intrinsic value with another item with no intrinsic value - Gold is the real constant here not dollars.)
I don't get what gold has to do with fast transactions.
There's no such thing as intrinsic value, but there's no need to discuss it here.
It's being discussed
here and
here.