Problem with installing new RAM

My Dell workstation came with 4 GB of DDR2 ECC RAM. I replaced it with 8 GB (4GB x2) of non-ECC and it worked fine. I've been shopping for an additional 8GB, but all the affordable non-ECC I've found is for AMD computers, while my PC is Intel.

So I bought 16 GB of ECC RAM, but when I plug it in the computer just buzzes. The four sticks are the same make and model number, but the labels seem to indicate they're two separate batches. I made sure that each installed pair came from the same batch.

I think it unlikely that all four sticks, from two separate batches, would be defective, so what else might be the cause?

I checked my BIOS: it not only is set for ECC but seems to think the 8 GB now present is ECC.

The max RAM figure is outdated.
Dell has told me, and users on the T3400 board have said the same, that the actual max RAM is 16 GB (4x4GB), and the Crucial website RAM checker suggest I add 2x4GB. I currently have two 4GB sticks in place, and it works fine.

But your statement that Dell 0TP412 does not support registered or buffered memory bears looking into -- I'll check with the vendor who sold me the RAM that isn't working.

Here's my BIOS and motherboard:
Board: Dell Inc. 0TP412 A02
Bus Clock: 1066 megahertz
BIOS: Dell Inc. A09 06/04/2009

I believe that is not the most recent BIOS.

Layback Bear seems to have spotted the problem. When I passed his comment to my RAM supplier, I got an ambiguous acknowledgement that seems to indicate this RAM is either buffered or registered. So I am returning all the RAM to him.

Thanks, all, for your assistance.