The short answer is not really. For copying via memcpy, the answer is no, because memcpy does not assume memory is aligned on an 8 byte boundary. So it only copies one byte at a time, as any memory alignment can be passed into memcpy.
For other operations, yes those operate on 8 bytes, but they do so for a single data type, such as a 64-bit integer. To operate on two 32-bit integers, you need SSE2 or the very least the older MMX instructions.