AVR-Chat

This is probably in the data sheet but I can't find it or maybe don't 
understand it.  I'm working on a project with two Mega324p processors.  
One is running at 5 volts and the other is running at 3.3 volts.  I am 
planning to communicate between them using the built in serial ports.  
Do I need to do something  between the two ports or I can I connect the 
serial ports diretly together?  I'll probably use a 1K resistor to 
protect the ports but am not sure if I have to reduce the output from 
the 5 volt processor to 3.3 volts.

Thanks for any suggestions.

Bob