SD Datalogger for serial device


I have a serial device that communicates over RS232 and am trying to store that data feed into a txt file on a microSD card.

I purchased:

Arduino Mega 2560 RS232 shield
Arduino Ethernet shield with microSD slot
Sparkfun microSD shield

in hopes one of the microSD shields would work.

My question entails a couple things. I assume I can not just plug the RS232 serial device directly into pins 18 & 19 and that I need to use the shield to make sure the correct voltage is applied and does not fry the board. If I stack the RS232 board on top of the ethernet shield w/microSD will I be able to use pins 0 & 1 for the RS232 feed or will these pins be taken up by the ethernet shield? I read that I should use hardware RX/TX pins if possible so that is why I was trying to stay away from putting the RS232 device on like 2 & 3. Maybe I need to rewire the RS232 device so that the RX/TX pins connect to 18/19 on my mega. Any help or suggestions would be much appreciated :).