With un-buffered sticks, 256MB DIMMS are the max supported.
I've heard rumors of people getting registered 512MB DIMMs to work in some 440BX boards (to a max of 1GB), but I haven't tried it.
I'm sure I've got some buffered dimms here somewhere. I might have to try it on my BH6 over the weekend.
edit: Tried a single 512MB registered stick, and the BH6 only detected 128MB. 😜

That pos system looks pretty nice. Makes me wonder if registered 144 pin so-dimms even exist...

This page should be a good read on the subject:
RAM modules compatible with motherboards based on the i440bx chipset

I ran a Soyo 440BX board for years with 3x128MB, and currently with 2x256MB. The only adjustment I made was installing the Win98SE unofficial service pack.

Yes, I had a DFI P2XBL with 2x128 + 256 , but never 2x256, so it is possible.

Mine gets up to 655360k (BIOS limitation?)

Is running the 440BX with 1GB RAM total not sufficient for your application/desires? If there is anyone else like me out there who still uses a 440BX as their everyday computer, I suppose a configuration with 2GB total would be desirable. When I have Firefox and Chrome open with a lot of tabs, it becomes very obvious when I've used up 1 GB of RAM (the machine slows to a crawl). As a side note, I plan on finally upgrading my 440BX system after constant use since 1998. I'm hoping a dual Tualatin 1.4 with DDR RAM will be the replacement.

well it only has 1 single so-dim slot so at max it can take 512mb if i get lucky. we are gonna try and work with the 256mb limitation and strip down windows xp services and run with out anti virus sine they won't be used for internet surfing. The people that make the software dont' know the difference between mb and kb and also told me 256kb and 512kb isn't even enough to boot XP, and by kb they meant mb and i have seen xp run on 256 and 512 so i think when it's all said and done it won't be unusable but, it won't be fast either.

also i'm going to read gerwin's link.

I'm running XP PRo on an 430TX boad with 256 MB RAM and a K6-III-450. After boot, XP consumes 78 MB of RAM when idle. Most webpages will bring the RAM usage up to 240 MB pretty quickly; that's just with one open tab.

well thankfully this is just POS software but, it's current so who know how much ram it will use with out actually testing it.