here's a good bit of info from an article over at
Tomshardware.com the athlon was said to transition to the T-bred(a) from the 1700+ to the 2200+
and the better T-bred(b) from the 2400+ upward
in actuality tho you will hardly find any T-bred(a)'s under 2200+ (i've heard a few reports of 2100+ T-breds)
but basically any 2200+ you buy is guaranteed to be a T-bred core
and any 2400+ or up for now will be T-bred(b)
most everything else is gonna be a .18 palomino unless stated otherwise
have fun