Okay.  The six-pin AT-style plug has extra 3.3v on it, I think for some older SMP boards.  The "P4" connector has extra 12v.  The 20-wire alone has relatively little 12v on it, which made sense when the processor got ran off the 3.3v or 5v rail.  Now the 12v line's used.

 

Although called a P4 cable, it's also called for by some later Socket A boards and I think A64 boards also.  You may leave it disconnected if the mobo has no socket for it, or you could probably rig a fan or something for it

 

With a motherboard which doesn't need the P4 cable, look for a lot of 3.3 and 5v.