What stops the banks from plopping down 250 million dollars worth of hardware to attack the bitcoin network?
If one of them does that, their intelligence network would already have propagated that to other big banks. If several ones do that, it's just fine ^^
Also, hardware isn't available instantly, it needs to be produced first.
Hell what stops them from pumping in 1 billion into a research project to develop a new chip/system for themselves ...
have you heard of
http://mintchipchallenge.com/ ?