North bridge is located on CPU, correct? So if I drop a Xeon in an Optiplex 980 I can use ECC memory?

Optiplex 980 board uses the Q57 chipset, which doesn't support Xeon CPUs. It would have to have the 3450 chipset to support Xeon.

A016 bios update allowed Xeon support, I know it works - That aside, Northbridge on CPU = max ram/ecc CPU bound?

OK, I found a different resource. Looks like if the BIOS update supports Xeons (assuming 3400 series), then the Q57 does say it will support ECC RAM (DDR3 1066/1333) with a Xeon 3400-series CPU.