I was going to recommend considering just returning the new memory while you still can(since, I assume you haven't opened the packaging yet) and look for a replacement motherboard instead. But while plausible that it is the MoBo the new memory might end up coming in handy anyways.
Your best bet is to try replacing that motherboard however. The fact that it is a dell OEM system with what is likely a custom layout MoBo with custom ATX power connector from the custom PSU into the custom MoBo tho...
You are likely going to end up looking at getting a whole new system anyways. This is why these big brand name workstations aren't all that great unless you specifically buy into extended warranties and just leave it to them to maintain and fix/problem solve.