"10.5 is the last PowerPC compatible mac os. That is the max OS supported. I believe it wants a 866mhz G4 or better and a DVD drive, but it could potentialy work on slightly slower machines. 10.5.8 is the most current version.
Now I had it in on a 933mhz g4 tower and I found it too be on the edge for acceptable use. It would depend on which imac version you had to further know what your user experience would be like. I think it is ok to use as long as you turn off some of the visual effects to speed things up (dock magnification, transitions, etc.)
http://en.wikipediadotorg/wiki/Mac_OS_X_v10.5
Apple states the following basic Leopard system requirements, although, for some specific applications and actions (such as iChat backdrops) an Intel processor is required:[51]
Processor must be any Intel, PowerPC G5 or G4 (867 MHz and faster) DVD drive (for installation of the operating system) At least 512 MB of RAM (additional RAM (1 GB) is recommended for development purposes) At least 9 GB of disk space available.
Leopard’s retail version was not released in separate versions for each type of processor, but instead consisted of one universal release that could run on both PowerPC and Intel processors.[32] Leopard drops official support for slower G4 and all G3 processors.[51] Because all new Macs use Intel processors, the versions of Leopard shipped with them are Intel only."
OS 10.5 should be perfectly fine on a G4, although sometimes things like speech recognition can stumble a bit. I've seen this on a G4 power-book so I assume an iMac would fare even better
