Yes, it's a downgrade from an Athlon XP. The Sempron's are the new Duron's if you like, the budget AMD chip. XP's will disappear shortly and you'll be left with A64's on s939 and Sempron's on s754.
The Semprons rating is to compare them to the Intel Celeron. So a 2500+ Sempron would compare to a Celeron 2.5GHz (if one exists!).