Choosing cpu type when building kernel (stupid one i know)

[Steel City Phantom]

in bsd 6, i have to recompile the kernel to add atapicam.  ok, fine.  
the part of the config file that gets me is this:

machine         i386
cpu             I486_CPU
cpu             I586_CPU
cpu             I686_CPU
ident           GENERIC


the machine i386, i get.  all intel PC's since the 386 have used a 
common basic instruction set and the developers just want to build in 
something in case intel pulls an Apple Computer on us. 

the rest is what i don't understand.  486 isn't mentioned in the NOTES 
file, 586 is just pentium, and 686 is pentium pro.  I have a P4 1.4 gig 
intel chip in my system, what should these settings be?  if i remove 486 
and 686 does that give me any performance advantage or are these 
settings defaults because they are ment to be defaults and i should just 
be quiet about it.