If you have the available RAM/SSD for a birthday attack, you only need 48 bits worth of complexity, which makes this a manageable problem. I suppose the ASICs for bitcoin mining don't have the needed bandwidth to storage devices which would put my guess onto GPUs. Assuming 32 bytes of storage per hash (probably more but you wouldn't store the full hash, instead a hash table with the prefix+postfix up to a certain amount, then of course some hash table management overhead), you'd have to write around 9 PB if you write once, 1 petabyte if you write 64 times, for the raw hashes only.