Can anyone tell me the rules to validate a credit card number?

I need to pre-validate a file of credit card debits before we upload it to avoid having to...

- stop the upload
- edit the text file to remove the invalid card number
- re-start the upload

I know that numbers begining with 4 are MC (or Visa) and numbers beginning with 5 are Visa ( or MC :) ). Is it true that all numbers must have 16 digits and are they always 4 groups of 4 numbers?

Any help is appreciated...

TIA