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.