What's a good hash algorithm for hashing a short length (e.g. 256 bytes) buffer, which must be fast and very collision resistant? It doesn't need to be cryptographically secure.


Would CRC64 be a good choice?
What's the minimum hash length that can be considered cryptographically secure these days?
First you should check if a cryptographically secure hash such as SHA-2 is fast enough. It is likely it is fast enough, and then the fine details of your weakened collision resistance requirement become irrelevant. 
If you really need something fast you may consider CRC-64, but it is obviously not cryptographically secure, It is rather trivial to construct collisions for CRC-64. However if you are trying to protect against random collisions, it may suffice, and with random input data, you may assume CRC-64 won't collide much more then a random function which mean you can use birthday paradox to calculate the chance of having a collision. Essentially you need number_of_messages &lt;&lt; 2^(bit_length/2)