There are 2256 private key possibilities -- which is what's relevant. I'm not claiming successful collision is likely.
2
160 is the correct number as you only need for the address to collide in order to move/spend the coins. Any of the 2
96 key pairs that hash to a given address will do.
There have been dozens and dozens of threads on this subject (address collision).
If you pay your miners on address hits then you will never pay them.
If you pay your miners on being "close" to an address that has coins then you are paying them for worthless "close but no cigar" addresses.
Darn, my attempt to kill this thread has failed