Different cores are just different 'generations' of a certain chip. For example, you could buy an AMD 64 3700 Venice core, which is fast, but not as fast as a 3700 San Diego which has a 1mb L2 cache (as opposed to the Venice's 512kb).
WRONG!!! but close
the rating basically indicates raw power, so 3700+ is 3700+ no matter what way you look at it. that and the fact there is no 3700+ venice but there is a 3800+ venice. cache makes a difference but it does not effect the power of a given rating. for instance take the san diego email@example.com vs. the firstname.lastname@example.org. the san diego is 4000+ and the venice is 3800+.
the different cores basically define different features. e.g. venice=90nm, 512kb L2 cache (and some other features we won't go into) san diego=90nm, 1024kb L2 cache.