So the i7 920 comes with what looks like a nice heat sink with a fan on it.
I am guessing that this is fine if one does not over clock, and that over clocking requires a better CPU cooler, is that correct?
Also on the mobo are 12v pins for powering fans, when I add additional fans to my case is it better to use these 12v pins form the mother board rather than buy 5v fans and power them via the power supply?
Yes, if you are going to overclock the CPU you will need to add additional CPU cooling -- unless you want to hear a loud pop and open up your PC to find the CPU has exploded driving a hole clean through the motherboard!
Overclocking is an art and it depends a lot on the chip you have and the motherboard you are running. I suggest you check out some of the dedicated overclocking forums elsewhere on the Internet for a much more detailed description of what you need to do and how to do it.
The stock CPU cooler is not very good. You better get a replacement cooler like a Noctua or Thermalright.
There are a number of threads in this forum about overclocking the i7-920 on P6T mobo's. Search a bit and you will find plenty of info.
I would advise against using 5V fans. I would only use 12V fans, first using all mobo connectors and when you need more, to use conventional Molex connectors from the PSU. The airflow from 12V fans is much better than with 5V ones.