OP_0 pushes an empty array of bytes onto the stack, so you should RIPEMD160(SHA256([])).
(thanks go genjix for setting me straight, I'd been thinking OP_0 pushed a 0x00 onto the stack, and that isn't right. The scripting engine knows that an empty array is 'False', and FIPS standards make sure hashing empty strings/arrays is well-defined...)