I still don't understand why you can't really use a powerful CPU for mining. Is a GPU really more powerful for these calculations?
Short answer: Yes.
An Intel Core i7 2600K overclocked to 5.0 GHz can generate a hash in only 0.2 microsecond by running a single thread of Ufasoft. On the other-hand, the best VLIW5 GPUs (clocked at a mundane 850 MHz) take just over 4.0 microsecond to calculate a single hash.
So how can something 20 times slower yield such a large performance advantage?
This is where parallelism comes in. A Radeon 5770 / 5870 at 850 MHz (stock speed) can generate 800 / 1600 hashes every 4 microsecond. That Core i7 2600K (at 5.0 GHz) can only generate 4 hashes every 0.2 microsecond. Hyperthreading allows it to run 8 threads simultaneously but only increases hashing performance by 25%. So at best, this CPU will generate 100 hashes every 4 microsecond.
And of course the GPUs can be overclocked too.