4 fast cores or 6 slower ones? I have the option of 3.33 ghz 4 core processor, a 2.66ghz 6 core or a 2.93 ghz 6 core.
I know that the middle one will be slower than the last, you don't have to tell me that.
What I would like to know is what would work better? 4 fast cores or 6 slower ones? I know that if you do the math the last one is the best but how many cores can Blender use?

I would also like to know what would be the best for blender internal renderer and cycles (if there's any difference.)
Blender can use up to 8 cores, and it does.
For rendering, or baking the two most CPU intensive tasks, the 2.93 ghz 6 core would be better. The clock speed is not as important as computational power, and for that the more cores wins.

For Cycles a good GPU would be faster then any of those CPUs listed. However if you are rendering on the CPU (like me) then the same is true more computational power will win out over a faster clock speed.

Blender guru article on buying a computer for blender.