the Celeron D is a dual core CPU. the Sempron will perform about the same as the Celeron when running single threaded programs (will only use one core) but in multithreading programs, even being overclocked won't put the Sempron ahead.
however if you compare it to a Celeron (not Celeron D) 2.8, you can certainly overclock it and make it faster, provided your motherboard and RAM can also handle it