Passport Validity for immigration visa

[perfectp]

My parents will appear for Family based immigrant visa interview during next week at US Embassy in Mumbai. Their passport is expiring on 2nd December 2010. If they are issued visa, do they have to travel immediately?

Does 6 months passport validity rule applies to Immgrant visa?

After receiving Immigrant visa, can we renew passport before traveling?

I need expert guidance in this matter.