It ain't the RAM like the guru said; it makes annoying beep sounds for RAM, HDD, and Video Card.
Do you get fans spinning up? I can't imagine the difference in power consumption between two sticks of RAM would be great enough to make a PSU work with one setup but not another with the only difference being the RAM. But it's possible.
If not, check your VGA cable. A lot of systems have two VGA slots, one that goes to the integrated video on the mobo, and one to your graphics card. So make sure when you reconnected your wires after opening up your computer you reconnected it to the correct port.