Oh, boy...Intel D850GB and RIMMs - ECC or what?

Hello! I have a problem with an old computer, so here goes : I don't have any money for a PC right now so someone thought to donate this old server. It uses an Intel D850GB motherboard with a Socket 423 CPU and 4 RIMM slots. The system currently has 4x64MB PC800 Non-ECC RIMMS installed in it, to a total 256MB of RAM.
That may have been more than enough in the good-ol' days of 98 and 2000 but nowadays...
So I bought some RIMMS (2x256MB) off of someone and they are both rated as PC800 but they are different models, both from Samsung and both are ECC. The problem is that the system doesn't want to accept them in any way whilst his system, an old PIII Socket 370 works just fine with them.
I've tried using 1,2 or 3 of the other PC800 64MB RIMMs in all kinds of different combinations but to no avail. The motherboard just does the 3 beeps.

Is it because the 256MB RIMMs are ECC and I need to just populate the first bank and then use CRIMMs in the other bank? Is it because they are different models? One of them has the heat-sink one one side and the other has the heat-sink covering the whole RIMM so I'm guessing that one of them is single-sided and the other one double-sided (because one of them has memory chips one just one side of the PCB while the other has memory chips on both sides of the PC)?
I don't know, I'm just guessing right now.
I do know that in bank 0 I must use RIMMS of the same size, speed and density but in bank 1 they just need to be of the same speed. Guess that's not true because that was the first thing I tried and all I got was just beeps. But I'm repeating myself.

If anyone has any ideas, please let me know. I've spent the whole day trying to figure this out. I have a hunch that it's because I'm trying to populate one of the banks with ECC RIMMs and the other with Non-ECC RIMMS but I'm hoping I won't have to buy CRIMMs.