[JohnE.]

The extra wires in an 80-pin cable reduce crosstalk and provide some measure of reduction in data transmission errors (or something close to that description anyways). Even ATA33 devices will benefit from reduced crosstalk. The ATA specification at the time actually recommended 80-pin cables for ATA33 but it was not made mandatory like it is with ATA66 and higher.

I think it's Blue to Motherboard, Black to Primary Master and Grey to Secondary Master... at least that's the recommended setup.