How strong is the bitcoin network? Let's say an organization decides to attack bitcoin by launching a 50% attack.

Can someone estimate how big the effort is? 


How expensive would such an attack be from scratch (buying enough
ASICs)? 
How feasible is it for an organization with a lot of
computers to do it using their existing computational power?


For example, can a company like google do it by, say, turning 5% of their computers towards attacking bitcoin? 
Can the US government do it (if they chose)? 
Can a few banks group up, buy some equipment and hijack bitcoin this way?

I understand that these are all possible in principle, so I'm looking for back of the envelope estimates of how realistically feasible such scenarios are.
Three months is about the useful life of an ASIC miner. The cost of all practical mining (not including wasteful mining done by botnets and the like) tends to equal the value of all practical mining. So an approximate rule of thumb for the cost of an attack is the value of all Bitcoins mined in three months. Since the block reward is currently 25 BTC, Bitcoins are around $700, and a block is mined around every 10 minutes, you're looking at roughly $200 million. Using existing equipment is impractical -- you need ASICs.