Answer to: Mods for tiger?On the topic of non-native OS support. There is a tool out there that I remember reading about that will temporarily change the "processor speed" to make it compatible with leopard. I put that in quotes because it doesn't actually change it, but rather, it changes the cpu report in the NVRAM(? I believe), so that you may fool the Leopard OS disk into installing onto a Mac that runs slower than the 867mhz Leopard minimum. I cannot remember where I saw this tool, but I would recommend googling it and checking the Hackintosh forums. Beyond that, max out the RAM, inquire about a HD upgrade, keep your login items to a minimum and clean the computer regularly.
Answer to: hard drive - noise and clickingThe 7200 RPM drives are known for having loud noises when installed in laptops. That is, as I understand it, the reason apple does not install them from the factory. If the OS is pausing and you're hearing clicking, I would assume that there is a problem with the HD. Some HD's are just loud though. Best way to test it would be to mount another HD inside and clone (http://www.bombich.com/index.html) your system to see if the sounds and pausing OS problem continues.
Answer to: What type of OS is in my PowerBookIt depends when you bought it. Some PBG4's came with Panther (OS X 10.3), most came with Tiger (OS X 10.4), although it is capable of running Leopard (OS X 10.5). To find out which one you are running, see Tyler Bly's Answer. (Go to Apple Logo in upper left hand corner and select About this Mac from the drop menu).