|Did you make sure that the machine was unplugged when inserting or removing the memory?|
Chances are that you nudged the old memory, even if you did not intend to move it. It is possible that the contact is tarnished. Remove the old memory, clean the edge contacts with a clean pencil eraser, then pop it in and out of the socket 4 or 5 times to then burnish the socket, cleaning it as well. See if that helps.
for beep codes for your BIOS or google for the chart for your motherboard specifically.
You have to be a little bit crazy to keep you from going insane.