You should be able to install leopard anyway if you have enough memory.
imac 800 leopard install
"You will probably be able to run Mac OS X 10.5 on an unsupported machine with XPostFacto, but it may be too slow to be usable, and it will not be available on the Mac OS X release date."
change clock speed for install. see this post:
Jul 19, 2009, 08:45 PM #17
Mac Man 82