I am not a card keeper (unless it includes a heartfelt message) but a word of advise.......hold on to birth announcements you get in the mail! They will come in handy when you are about to visit a friend and can't remember his/her kids' names.
You might be wondering where you should keep stuff like this?!?! I have a photo box with memory stuff in it. It includes souvenirs, concert tickets, notes, birth announcements, etc. When the box gets full I go through it (which is actually fun) and purge anything that doesn't drum up positive memories. If I can't remember where I got it or why I kept it, in the trash it goes!