Upgrading to OS Leopard - cannot install on this computer??? Why??

I have just upgraded my ram on my ibook g4 1.33ghz powerpc. It now has the maximum ram for this model and over 30gb of hard drive space, however I cannot upgrade my OS to leopard, which according 'experts' I should be able to. I go through the usual steps and confirm I want to install in english and then a window pops up saying unable to install on this computer. Any ideas?? I am losing patience as I have spent out on the software and the ram and still seem to be getting nowhere. HELLLPPP!!!