What should I do with my original RAM after an upgrade?
Several threads on MacRumors say to keep it for troubleshooting and/or if you need to send it in for warranty service:


Upgraded from 2gb to 4gb - what to do with old Memory?
"2x1GB is worth so little so you're better off keeping it for troubleshooting purposes."
What to do with the old memory on the macbook
"I would hold on to the memory; if you ever need to take it in for service, it's better to have the original memory in it."
"Keep them for warranty"
what can i do with old ram? 
"Apple guidelines are that if you do have 3rd party hardware installed, they won't service it."