Replaced 128mb with 512mb in

Question

Replaced 128mb with 512mb in Dell Dimension 4400. Plugged everything back in & turned on. Or at least think it is turned on; there is a green light for both the power button and hard drive lights. Had flat screen attached and there is not light. So unplugged and put in a Dell monitor, the light goes green then yellow. I am not sure the 4400 is even on but the dvd and cd rom works and other lights are on.