To answer your question specifically, you can use our Mac ID system to identify precisely which MacBook Pro you have in your possession. At the end of the system, it will give you a link to all the parts that fit your specific machine.
However, changing the AirPort card may not solve your connectivity problem. Your AirPort should be able to discern between 5GHz and 2.4GHz devices, depending on how the base station was configured. Have you tried downgrading your AirPort Base Station to 802.11g speeds and seeing if it connects?
Peter, I still don't know for sure which model of MBP Unibody you have. What is the model number of your MacBook Pro? Did you go through the Mac ID system to figure out exactly what machine you have?
I'd love to help you out, but I don't want to tell you "hey buy this card" and then find out you have a different machine. My current *guess* is that you have a Mid-2010 MBP 15" Unibody, which would correspond to this AirPort install guide and this AirPort card. But, I can't tell you for sure until I know either your machine's -- the one you want to put the card into -- model number.