Upgraded RAM from 1GB to 2 X 2GB, won't start up, WHY?

When I upgraded ram from 1GB (factory installed) to 2 X 2GB, the fan comes on, no sound or screen. I took the one 2GB stick out and replaced with the 1GB back into the original slot. Computer starts up, recognizes both RAM (original 1GB and new 2GB), everything works. Any combination seems to work, slot placement and RAM configuration, as long as the original 1GB is used. I could understand if the new RAM was not recognized but it is. What is the problem or what am I doing wrong?