Adding Memory to Dimension t500

I've tried to add two 256MB memory cards to my Dell Dimension T500 but the PC fails to start afterwards.  I get "Memory failed to be sized or enabled" diagnostic code.  I've done some basic checks.  The existing 128MB card works in each expansion slot but neither of the two new cards work in any slot, either with or without the original card.  BIOS is up to date, although it seems old ~2000.

Original card is PC100-222-620; new cards are hynix PC133-333-542, 256MB 133Hz CL3 ECC.  Handbook say I should be able to expand to 768MB providing memory is ECC compatible.  I also understand 133HZ is backwards compatible with 100Hz.

Any suggestions how I can make the cards work or explanation why they don't work so I don't get it wrong again.