Most important spec to look for on a GPU for scrypt mining (e.g. Litecoin)
I think the important factors for scrypt in OpenCL are as follows


GPU core clock rate
Memory clock rate / memory bandwidth
Memory bus width / channel
Computation Units
Processing Elements


You can check out in Programming Guide of AMD Accelerated Parallel Processing.

single or double precision of floating number does not matter. Either SHA-256 or scrypt is doing integer arithmetic operations.