2 GB Ram is not working

One thing that always should be done to prevent damage to parts plugged into a motherboard is to pull the power cord out then press the power button, or wait a minute. Motherboards of the last several years always have power to them when plugged in, most have a small light on the board to indicate that. Removing things like the RAM modules and PCI cards can cause damage to them if the residual power is still present.

Back in the AT motherboard days the power switch was connected directly to the power supply, the newer ATX/mATX boards control the power where the power supply is connected to the board and the power switch on the case is also connected to the board and only tells the board to turn things on or shut down. Or in other words the ATX boards control what the power supply does, not the case switch.

Your last post implies that the new 2 GB of 800 MHz RAM worked. You say "it was working" and only failed to work after you uninstalled it and then reinstalled it.

Your first post implies it never worked.

So I'm confused.

The manual Carwiz links to in post 9 says this:

"Module types supported: 512 MB, 1 GB, 2 GB (800 MHz not available in 2 GB DIMM)."

Are you trying to use a single 2 GB stick of 800 MHz RAM??

Or two 1 GB sticks?

In your first post, you say "when I installed it in DIMM 3"...

That implies you have just one stick. A 2 GB stick.

According to Dell, that is "not available", which I take to mean "not supported".

So, I wouldn't expect it to work.

But maybe we don't understand what you have or what you have done.