Why my iMac don't startup correctly?

Hi

I have a big problem, my iMac wont startup. The led n°1 and 2 glow but the third keep closed. I have recently changed my RAM. I dont have chimes and the fan is in full speed. I do all the things who the internet say but isn't work. My USB port work.

iMac Intel 20" late 2006 Core© Duo with 4go RAM PC2 6200S