I wonder why AMD Dual Athlon 3600 is much more expensive than AMD Sempron 3200 whereas the clock speed of the Sempron is 2.0 GHz and the clock speed of Dual Athlon is 1.9 GHz?
Some ebay sellers multiply any Dual processor by 2 to get the clock speed for example for the Dual Athlon 3200, they claim 3800GHz or 1.9*2. Are they right? Can I say that a Dual is actually 2 processors and not one?
A dual core CPU has two processing cores in it, just as a quad core CPU has four processing cores.
Two processing cores is NOT the same as one core running at twice the speed. If you have two threads which can run in parallel, you may get similar throughput, but if you are running a single threaded application, then two cores is no faster than one.
There are further complications. You cannot judge the performance of two CPUs based solely on clock speed. Cache size makes a difference. Even more important, some cores are more efficient than others. For example, Intel Core 2 core is significantly lower performance than an Intel Core i7 core at the same clock speed. I don't know if the Sempron and Athlon cores compare likewise, but I wouldn't be surprised if that were the case.
Sempron is the AMD value line. Think Celeron on the intel side.
As said above, you can not add the 2 cores. Many e-bay sellers sucker people this way.
If a program can take advantage of more then one core(lets say Starcraft 2), there is a boost, but if it can not(lets say ummmm hmm... angry birds, yeah that will do), the program will run as if the system only has one core.
Please make sure when getting a new cpu that your board of choice will run it as well.
There is a cpu chart on this page, it does not show the Sempron because they are just not that fast. It just gives you an idea of where each cpu sits. As said above, clock speed does not mean as much any more.