I did a quick google search last weekend and couldn't find an obvious answer so I spent some money to find out empirically if buffered ECC memory works in a 440BX motherboard. I tested on an ASUS P3B-F motherboard with a Pentium III-S CPU. The manual says it supports ECC memory, but doesn't distinguish between registered and unbuffered.

I used two double sided (9 chips per side) 256MB sticks of registered ECC PC133 SDRAM with these codes on the sticker
Micron
MT18LSDT3272G-133E1
PC133R-333-542-B1

What I found is that these behaved like high density 256MB sticks in a 440BX motherboard. The BIOS and Windows could only see half of each stick. This was an unexpected result. I was expected them to either not work at all, or work seamlessly. My guess is the registers on each stick present themselves to the memory controller in a way that appears like a high density stick, thus the behaviour.

Performance was identical to regular CAS3 PC133 SDRAM. You need to flip a switch in the BIOS to turn on ECC mode (see manual to find the setting).

We could probably verify my theory about the register by using a 128MB buffered ECC stick and see if the whole thing can be seen in the BIOS. I don't feel like spending any additional money on this, since I don't think ECC memory adds much value to what most of us are trying to do with our vintage computers.

If anyone happens to have a 128MB registered ECC stick laying around and wants to try, please post your results here for the sake of knowledge.