Seven year old software is ancient in computer terms. PowerPC ("PPC") is the type of processor. The last of them was the G5 that ceased production in July 2006.
Here are the REAL facts....
-- Some apps CAN run on either Intel or PPC.
-- Some PPC Macs can run OS 10.6.8. I am typing this right now in OS 10.6.8 on a dual-processor G4.
-- Most software installers (and most software CDs / DVDs) only have one install option: Windows or Mac OS. You cannot use the same CD / DVD for both Windows and Mac OS.
Here are the imperfect or confused statements....
-- "PowerPC cpus have a lot of compatibility problems with software and operating systems" NOT! Either the software app will install and run or it won't. There is no compatibility problem. Same with OS versions. Either it will install and work fine, or it won't install at all.
-- "Starting with I believe 10.5, PowerPC apps no longer run on an intel mac." Most Mac apps developed in 2006-2008 are "binary" development. That means they run on either PPC or Intel. The app has dual code. The special software that Apple developed (and that is included with OS 10.4.6 to 10.5.8 -- optional installation with OS 10.6 to 10.6.8) to enable dual-processor compatibility is called "Rosetta", like the translation stone in Egypt. OS 10.7 has no option to install Rosetta software.
-- "Macs are supposed to be backwards compatible" You don't really expect that. The first Mac was in 1984. No statement from Apple has ever claimed that Macs are backwards compatible. Apple always explains the compatibility of each software product on a case-by-case basis. You cannot use iTunes 5 with OS 10.4; you cannot install OS 10.7 on a single core Intel Mac; you cannot install OS 10.4 on any Intel Mac (OS 10.4.6 or later for the first range of Intel Macs).
If you have PPC software, read the info on the package, but it likely will run on any G4 or G5 Mac.
As to Apple switching from PPC to Intel, it wasn't really related to performance or to using Windows. It was related to heat. There was no G5 notebook because the processor ran too hot. The trend in the past ten years has been an ever-increasing slope upwards for notebooks. Who do you know now who is buying a tower? Not one person in ten. If Apple hadn't switched to Intel, they would be nearly bankrupt by now.