Question: Suppose organization X has 51% of the hash power for a period of 1 week. In this week, what exactly can and can't X do?

Answer: 
Anyone who owns 51% of the network will have made a massive investment in hardware and systems to organize and construct a machine capable of executing such an attack. If their motive is profit, then the short term gain associated with forking the block chain to enable 'double spend' will net them a negligible benefit; it's difficult to imagine they would pursue this strategy on the basis that the resulting instability will ultimately de-value the very coins they seek to 'spend twice'. If their motive is to destroy Bitcoin, period, that is another matter altogether. That kind of [techno-vandalism](http://www.yourwindow.to/information-security/gl_technovandalism.htm) could only reasonably be motivated by someone with destruction and disruption serving as their primary motivation.

Instead, I posit it's much more likely that such a massive and powerful compute resource (Bitcoin supercomputer) will be used to power the vast bulk of the network within the bounds of its intended use, profiting long term from generation rewards and transaction fees, as the network grows and prospers over time.
A 51% percent attack would simply destroy the currency, and anyone holding a short position would make 100% profit. So there is a strong financial incentive for this attack and it has happened many times.