It sounds like you can update the OS to Snow Leopard or Lion, and an iffy HD (if that's what that noise means) is cheap and easy to replace. In my case I can still use a G5 iMac on Leopard without exposing myself to the majority of problems out there. But it's slated to be replaced sooner rather than later, if only because it's getting long in the tooth. For business requiring up to date security I use a more recent MacBook and OS version with updated software.
