the sempron is essentially an Athlon64 with a much smaller L2 cache. it can run at it's rated speed, but due to the smaller cache, it cannot access as much data as quickly as an Athlon64.
a sempron is a budget CPU designed for more basic taks. it may run some games fine, but it is not designed for it.
Semprons are like Intel's Celerons. Even though the clock speed might be the same for both, Semprons are much slower than Athlons because they run more instructions per cycle.