Sempron 2500 is the same speed as a 2200. It's just a rebadged Athlon XP Thoroughbred core 2200.
The 2200 rating is comparing it to a P4A, the 2500 rating compares it to a Celeron D.
Sempron is the budget CPU, so the ratings are tuned to compare them to the Intel budget CPU - the Celery(on). Yah, it's confusing and it stinks...
Just make sure you have the jumper set to the right fsb speed for it (166)
You may be able to get it to report as a Sempron 2500 with a bios update, don't know for sure though.