A couple of months back I upgraded to OS X Panther on
my 400MHz iMac G3, but since then I have only been
running it on the minumum 128MB RAM. To overcome the
slowness, I bought a 512MB PC133 64bit 168 pin SDRAM
DIMM off ebay, but when I tryed to install it today the Mac
would not boot, and gave a simple beep rather than the
usual startup bong. I tried resetting the PRAM and CUDA
switch, but to no avail.

What gives? As far as I understand, slot loading iMacs
made since 2001 should accept PC 133 modules
(according to Apple), up to 512MB in each slot. Any ideas?

It has to be 133/100 pure 133 will not work. Also boot to 9 and make sure your firmware is upgraded.

macs are very fussy about Ram, and the ram you have
bought, although system compatible is specs, is not
"manufacture" compatible with the Mac, you are better
buying from Crucial.

I had a similar prob with my imac 333. Not sure if the boards are the same...anyway if they are you have to make sure the card is seated all the way. This requires some force more than you may think,I replacced my RAM and the same thing happened. I freeked out took it out replaced it and still the same. So I broke down and paid the mac tech to look at it. took him ten minutes...time it took to take it apart and put it back together. the card was not seated all the way. I would suggest applying a bit of force to the card to be sure it is reseated...My understanding is that it can withstand a lot.