Tom's Hardware did a whole article on the difference between a TBred A and a TBred B. Start on This page and read through.
http://www.tomshardware.com/cpu/2003...2_to_june_2002
The only real difference it shows is the B has one more layer, and is capable of running a core clock speed of 166Mhz, vs. the A's 133. This equates to the difference between using DDR 2100 and DDR 2700.