I recently purchased a used powermac g4 from a local computer tech school that went out of business. I've never owned a mac before but was interested in learning about them. The powermac came with the cpu, monitor, keyboard, and mouse. No CDs or "extras" When i turn on the mac all i get is a gray screen with a flashing question mark/mac face. Now to my question...if I buy the OS X Tiger from the apple store....will it install and is that all i need? or is there an OS on my mac and i just don't know it? Somebody PLEASE reply.....I'm at my wits end....Thanks