How much cash can I take out of Nigeria?

You should be concerned about your destination as much as Nigeria.
Most countries put a limit on the amount of cash you can carry on a flight. I hear the US is $10,000; UK £10,000.
You will be required to make a declaration to the Nigerian customs should you have more than $10,000 on you; failure to do so can land you in problems.
It is advisable to have the money transferred to your foreign currency account.