The minimal number of transactions per second that you need in order to keep the system secure is N = E / (60 * J)

So for SHA-256 (in fact, what hashing do you consider?):

• Let's take the Core 2 Duo hash rate for Joe
  J = 2.5 MH/s
  
• Today's hash rate of the Bitcoin network is around 430 PH/s. It is plausible to assume that a single entity owns 1% of that hash power
  E = 4.3 PH/s = 4 300 000 000 MH/s

 => The minimal number of transactions per second is the astonishing N = 28 666 666

Did I misunderstand something?