You also have to think of future updates to iOS. Each update moves them closer to not being able to sync with PPC compatible versions of iTunes, even if you have an older iPhone or iPad.
PPC Macs are great, but using them in conjunction with iOS devices isn't a great experience. You won't be getting even half of the features they can work together. As others have said, for the price of a G5 Power Mac, you're better off just spending that on a newer Intel Mac.
If money is really tight and you just want an old Mac to play with to get a feel for OS X, I'd recommend a G4 Power Mac. Those can be bought dirt cheap and the last few can still run 10.5.8 Leopard. They're a little slower than G5 Power Macs but far more reliable. They're also upgradeable. Look at the Dual Processor Quicksilver (QS) and Mirror Drive Door (MDD) models that are 1.0 Ghz and faster, they can all run Leopard. You can often find these for under $100, even $50 or less if you're in the U.S. (check Craigslist and save on shipping). Most people look at them and see an old computer, but they are great machines.
A good resource to learn what the latest version of the Mac OS that each Mac can run is
Everymac.com
10.5.8 is the last version of Leopard, which is the most up to date version of OS X that PPC Macs can run. But since then we've had 10.6 Snow Leopard, 10.7 Lion, and 10.8 Mountain Lion. Next summer we'll get another new version.
If you click on one of the more common G5 Power Macs, the first DP 2.0 Ghz model, and scroll towards the bottom, you'll see the version of OS X that was installed on the machine when it was first released, and the last version of OS X that can be installed on it...
http://www.everymac.com/systems/apple/powermac_g5/specs/powermac_g5_2.0_dp.html
In this case it's 10.2.7 when the Mac was released and 10.5.8 is the max it can do. You wouldn't want to run anything older than 10.4 Tiger on it, at least most people wouldn't, because that's when many new features came in. But Leopard would get you the most up to date software and apps, no matter which PPC Mac you buy.
Also note that the first few Intel Macs can not run the latest version of OS X, 10.8 Mountain Lion. Even the first MacBooks cannot run later than 10.6 Snow Leopard. Lion and Mountain Lion are the versions of OS X that offer the most compatibility with iOS devices.