[markem]

A password that is 6 characters in length and is numeric has (10^7)-1 possible passwords. If you have a 2.4 GHz processor capable of generating one password in, say, 100 instructions, then about (2.4 x 10^7) / (10^2) passwords per second can be generated. Testing them to see if they are correct depends on the user interface.

Most people opt for the minimum on their ATM card, which is usually 4 digits. Doesn't make one feel too safe if the ATM card information (not the card, just a skimmer) is grabbed.

One doesn't need parallel processors. All the math in the crypto is interger-based, so a graphic processor is way faster. Take a look at the NVidea CUDA tools for crypto processing. A few GPUs and most crypto looks pretty lame.