I don't have any first-hand experience with reg. ECC on 440BX, but here is an extensive guide that might answer some of your questions: https://web.archive.org/web/20180103032407/ht … ram_bx_faq.html

Yes, 440BX supports ECC. I have Dell GX1 in stock condition, it comes with ECC from the factory.
p.s. Here is a screenshot of ASUS P2B manual, it supports 72bit SDRAM, that is ECC memory:

I have two sets of dual-rank double-sided 256MB of dual-rank double-sided PC133 memory; one matched pair and one matched set of three. All of them work fine up to 512MB but adding a third stick results in only 256MB of memory being recognized. With just two sticks all 512MB is recognized.

After some experimentation I found that this was caused by the FSB being 133 Mhz. If I drop the FSB to 124 Mhz or lower then all 768MB to becomes available.

I'm guessing that a set of registered memory would allow these higher capacities on the faster bus speed as it would decouple the memory from the FSB.

Actually... further testing has revealed that the full 768MB is visible to the BIOS regardless of the FSB if I perform a warm boot. The value is always reduced at the higher FSB speeds on a cold boot.

I wonder if this is related:

http://tipperlinne.com/p2b-dsvio.htm