Using just A-Z, a-z, 0-9 and two more to pad out to 64 characters means (2^6)^8 = 2^48 tripcodes. Each one needs 12 bytes of storage if you sort them listed by hash (6 bytes if you keep them listed by password, but that makes for really slow searches), so 3072 terabytes. Using only lowercase a-z takes 2.3 terabytes, or a little bit less if you encode your data a bit more cleverly.