These are collectors items and cannot be used for anything. They can be usable if you spec it up! But even then they are like crappy G4 system...Most common G3's are: The PowerMac Blue And white, iBook Snow and the iMac Bondi Blue. iBook clamshells are getting more rare by the hour. You are really lucky if you find one under 150$. The biggest clock speed for the G3 is 1 GHz and those machines are really rare. To conclude, if you are not a collector G3 machines are not for you...
This is the sweetspot for all PowerPC machine's. They are no too expensice (around 50 - 100 $) and come with a lot of choices when it comes to clock speeds, ranging from 500 MHz all the way to 1.8 GHz. There is almost no rare G4 computer right now, they are all really cheap and can be found online. The most common G4's are the: 1.4 GHz single core proccesor PowerMac, the 800 MHz iBook G4 and the 1 GHz PowerBook G4. They are usable for light web browsing and any computer that can handle tenfourfox and 480p video can now handle YouTube thanks to QuickTimeEnabler! By no means can a G4 be used as main computer. It can not stream video or do any intense flash. Most of G4's are used as server machines because they are more than capable of that
G5's are really capable machines (for PowerPC's atleast), they can run most games and YouTube can work at 360p. They had clockspeeds from 1.4 GHz to 2.8 GHz. But there is one interesting things about them, they were the first quad-core machines. The G5 had only 2 machines : The PowerMac G5 and the iMac G5. The iMac G5 was the first iMac to feature the iconic design which is a lot of iMac's still use today. None of these machines are rare to find and are as expensive as the G3's (100-150 $). If you are a professional on a REALLY tight budget or if you are looking for a cheap space heater (haha, its just a joke) The G5 might be for you.
The PowerPC line of machines might not be the best option for everyone but if you are a student looking for a cheap laptop for writing notes in class, these machines might just be for you. Mostly because The PowerBook G4 and iBook G4 have amazing keyboards that a pleasure to type on! Besides that PowerPC's are not really the best bang for your buck....You can get MacBooks for nearly the same price and they can do soooo much more than a PowerMac. I am not trying to degrade the PowerPC platform i just dont want people to buy machines that can't do what they wanted them to do.