there are really only 2 different cores that were in XP-m's.
the first core tops out at 2.3GHz no matter how much voltage and crazy cooling you throw at it. it hits a brick wall at 2.3GHz stabely.
the second core are the famous ones that you see hitting 2.5 even 2.7GHz on air, sometimes on stock voltage!
i suggest you buy a 2500+ or 2600+. ill break it down for you
2600+ - 95% chance of getting good core
2500+ - 75% chance of getting good core
2400+ 45w - 40% ""
2400+ 35w - 60% ""
2200+ - ~20%?
you kinda get it, AMD bins their processors much like intel, except the mobiles have to go through another test to see if they can operate at super low voltages. usually if a chip can do a high speed on a low voltage, there is a lot of headroom to go even higher.
i would stick to a 2500+ or 2600+ just to kinda give you that sense of youre getting a special chip. its not much more i think.