If you are using a really slow disk the 80 conductor won't help much.
The difference are the extra 40 wires. They are not connected to anything, they are just to make sure there is less interference between the connections so it can run at higher rates. 1 of those 40 pins is used to tell the motherboard it is an 80 cable and not a 40.
TheMajorMMX - Intel P200 MMX @ 225Mhz - V-Tech (PcPartner) Baby-AT mb w/ Intel Triton TX chipset - Nvidia Riva TNT2 M64 w/ 16MB - 256MB M-Tec SDRAM - Still running.....