Precision T-3500 T-3500 uses DDR3 1066 MHz & 1333 MHz (Both ECC and Non-ECC)  Which slots are you using for old and new RAM? Sure the new RAM is fully compatible with your Precision T-3500 and with the existing RAM, including Rank?  Does the T-3500 boot if you remove the original RAM and install only the new RAM, and in the correct slots?     
I have a Dell Precision T-3500 with a 6 core Xeon X5650 CPU and 24 GB of RAM. I’m using it as a VMWare host, so the more RAM, the better. Recently I tried upgrading it to 48 GB, but it wouldn’t POST with the extra RAM installed. Dell’s literature says that 24 GB is the maximum for a T-3500, but at my last company, I had one just like this (also running VMWare) that had 48 GB and was working flawlessly. Any idea how to make that work on this machine, or if it’s even possible?
Precision T3500 Memory Upgrade Failure