Really having memory trouble.

I've got a IMAC G3 333mhz tray loading. I've figured things out a bit since I originally made this post. The memory I have fits but it is 512MB and I've read only 256mb per slot will work. What will happen if I put the 512 in? Will it fail completely or just not recognize 256MB? Also where is this second slot? I know that only low profile memory will work there but where is it? At first I thought maybe under the board but that doesn't seem to want to come off even with all of the screws removed. I need to have 512MB in it as my son put MAC OSX 10.3.9 on it and the 160MB it has just isn't enough. It can just barely function.