What pattern of LEDs (amber/green, ABCD) is showing on the back of the system?

A and B yellow, C and D green

Reseat the memory modules. If that doesn't cure it, power off, unplug, hold the power switch until the pilot LED on the mainboard goes dark, and retry.

Still nothing? You need to replace the RAM.

Hi my Dell 8400 would not boot the other day after switching out the processor none of the lights in the back of the unit would light up and the power button would stay amber color and the system fan sounded like a jet taking off I never realized how high these fans could spin up. Well anyway getting to the point I did a google search for dell no boot and I found something like my situtation in the Toms Hardware Forum. They told this poor guy to buy a new mother board, processor, system fan, Hard Drive, told him to change out his PSU and they even blamed Dell for it as an on going problem of this sort with there systems. Well this guy went ahead and did most of these things it seems he got it running. In my case the first and only thing I did was replace the heatsink I purchased a new and installed it and everything is working fine. Don't get me wrong these other forums like Toms Hardware are really good and they help out alot, I find that they are very good for people who build there own systems they tend to frown on Dell owners for owning a company made unit. I like most Dell owners preffer Dells forums because we use Dells and know how to deal with an issue if it should arise Thank you Dell for letting us post on our very own Forum