Before October 23, 2012, all iPads supported a minimum of Mac OS X 10.5.8 for connectivity and iTunes 10.6.3.
This meant they could hook up to G4s and G5s that have USB 2 and Firewire, as long as 10.5.8 was installed on them.
Starting September 2003, G4s and G5s came with USB 2. USB 1.1 available in older Macs only transfers data at 15 Mbps when nothing else transfers data, and frequently the USB 2 cards available did not have enough power to power the iPad when connected. 10.5 can be updated free to 10.5.8. For more on upgrading to 10.5.8, see this user tip.
Generation 1.0 iPad is compatible with Mac OS X 10.4.11 and PowerPCs that don't have the minimum 867 Mhz on a single processor, if the USB 2 card supported them. Generation 1.0 iPad can't be updated past iOS 5.1.1.
At this point the other main difference between Generation 2 and Generation 1 iPads, is the addition of the camera, and the preinstalled iOS of 4.2.1.
iPad 2 and later can all be upgraded to iOS 7.
Starting with the October 23, 2012 release (generation 4 and iPad Mini), iPads required a minimum of Mac OS X 10.6, thus making PowerPC Macs not able to connect to them. iPad 4th generation came with iOS 6. iPad Mini came with iOS 6.
Part of the confusion about compatibility is that iPad 2 and later can have iOS 7 or iOS 6 installed on them. Whether you can downgrade to an older iOS
is unknown, but may be essential if you end up getting an iPad that has already been upgraded past the top iOS 5. Be sure if PowerPC compatibility of any
sort is essential, you get those answers first before you buy a model with a newer iOS.
If you aren't running a minimum of Mac OS X 10.4 on your Mac, you'll have to buy 10.4 or to get 10.4.11 for the iPad 1, or Mac OS X 10.5 for iPad 2 and 3 as that can get you up to 10.5.8 free.
10.6 can be installed directly on a 10.4.4 or later on an Intel Mac, as long as you buy the retail Installer CD, and have a minimum of 1 GB of RAM.
For more on iDevice compatibility with Mac OS X versions, see this tip.