To also add to what has already been posted.
According to the technical specs on the AMD site TBred A's come in flavours from XP1700 - XP2200, Tbred B's start at XP1800 then skip XP1900 and range from XP2000+ upwards.
The Thermal Power and Processor Current is higher on the B's (example for the XP1800+ the typical thermal power is 55.7W on the B and 46.3W on the A and the typical Processor Current is 34.8A on the B and 30.9A on the A - note the XP1800+ shows the greatest difference for all the comparable speeds)
However for the Vcore it can be higher, lower or the same! Example the XP1800 for the A the Vcore is 1.5V but for the B it is 1.6V, for the XP2000 it can be 1.6V or 1.65V for the A but 1.6V for the B, for the XP2200 it is 1.65V for the A but 1.6V for the B! For XP2400+ and higher it is 1.65V.
http://www.amd.com/us-en/assets/content_type/white_papers_and_tech_docs/25175.pdf
The guide on the front page of overclockers is also very useful when ordering.