Dimension 4300 memory upgrade problems

Greetings.  I'm working on my in-laws' Dimension 4300 - and while I'm a very experienced system builder and modder it's thrown me for a loop.

Their system was still running on its original 256 mb stick of PC133 SDRAM and I offered to upgrade the memory since performance was no longer keeping up under WinXP. I purchased two sticks of 512MB PC133 SDRAM, ECC model. According to the Dell documents AND the Intel chipset specs, this machine supports ECC memory.
I swapped the memory in and got a series of beep codes from the system speaker. No POST, no BIOS flash screen, nothing but the tones (and the sound of the HDDs spinning up). I tried each stick individually in the main memory channel, and also tried loading one ECC stick in the secondary channel with the non-ECC in primary channel (which should just force non-ECC mode but otherwise start fine), and got no joy there either. System starts back up fine with the original 256mb stick back in the primary slot.
Both new and old are CL3 sticks.  Both the Dell documentation and the Intel 845 chipset documentation state that this mb should support ECC memory.  There are no relevant jumpers or BIOS settings available.  I'm using version A02 of the BIOS and plan to update later today, barring another reply, to version A06 (the most current, though it wasn' t successfully downloading yesterday).  The change notes don't say anything about adding ECC support in version A06.  Crucial says Dimension 4300 doesn't support ECC but this is where I trust that the guys who built the thing knew what they were talking about.  Memory density on the ECC units is 64x72; the spec on the replacement non-ECC unit is 64x64 so the density appears correct (just with the addition of a parity bit).

Additional diagnostic information: with NO memory installed, or any configuration with ECC installed, I get diagnostic lights (ABCD) YYYG.  Standard GGGG with OEM memory installed.  Beep codes with ECC are 3, 4, 1 (in that order).  Beep codes with NO memory are 2, 3, 1.  Machine still boots fine with the original hardware... if over 4 minutes from power on to my first app window appearing could every be called "normal!"

Looking forward to whatever you can share.