Dell dimension 4550 motherboard

Hello,I've just upgraded my 4550 to a single 1 geg ram card. The machine was working prior to the installation of the ram, it will not boot now. System leds on the back do not illuminate. The hard drive light comes on steady for about 45 sec. then extinguishes. Power supply is running and the voltages are nominal with output data supplied by on line searches. Has my motherboard joined the great programmer in the sky? Are there other checks I can do to verify a motherboard failure?