Arduinos use an AVR chip not a Propeller.
I do believe it's possible to overclock them, but I don't know how, nor do I know how much.
I can perform something on my Arduino that would take my GHz computer slightly longer. This is because the Arduino has much less overhead. If I were to bypass the Arduino libraries and program in assembly or whatever things would go even faster.
So clock speed just tells you how many cycles it goes through per second, it's related to the speed of processing but it's not a direct relationship.
You can have an Arduino interface with anything, such as another AVR, a propeller, a PIC, etc. Even my finger through the use of a button (and, thus, indirectly with my brain). 