I recently upgraded my ibook 800 g4 12" to a 1 Gig memory chip. I now have the 512 that I originally bought (in October 03) that I have no use for. It is PC2100 DDR type Ram. I am not sure exactly which machines this chip will work in, but I can verify that it works fine in my ibook.
The lowest price I've seen on 512 chips of this type is $89.50 (on Ramseeker.com) I'll put mine up here for $80 (US), shipping included (stateside).
Leave a message here, or drop me a PM if you're interested.