From your post, I gather that since installing Leopard, your iBook hasn't booted.
Leopard raised the minimum RAM needed for a mac to even attempt booting (I believe it's 512mb now, but I know it's more than 256, correct me if I'm wrong, people.)
If you meet the minimum, then try removing and reseating any RAM that's been added, as well as your Airport card. That may help. You may also simply be a rare victim of failing RAM.
Good luck, aegisart!