I have two computers, one with a sempron at 1.7 ghz and another with a celeron processor at 2.3 ghz, graphics and memory are identical. The sempron seems MUCH faster, any ideas why?
The word 'Celeron' is pretty unspecific, it's just the name that Intel gives to budget models of their Pentiums.
As for why the Sempron is faster, to put it how you might understand it, clock speed (Ghz) hasn't really been the determining factor for how a CPU performs for quite a while.
One Hertz (Hz), or 1/1000 of 1/1000 of a megahertz (Mhz, which is 1/1000 of a Ghz), simply just describes a cycle a second. Intel CPUs, while they may have performed more cycles per second, did not do as much work per cycle as similiar AMD CPUs, which is why the Sempron performs better.
This is also why AMD has shyed away a little from using Ghz to describe the CPU, and now uses series numbers. For instance, the '2500+' after the name of you Sempron means that the processor should perform at the same speed as a similar Intel CPU at 2.5Ghz.
Hope this helps you understand it a little better.