The farthest any PPC Mac (both of yours) can go is 10.5.8 which dates from Oct. 2007. Apple just brought out OS 10.8 three days ago.
Get more information about your computer. Go to the Apple in the upper left corner of any window, then "About This Mac", then "More Info..." Copy and paste the information here, but omit the serial number and Hardware UUID (if present).
compare with:
Leopard 10.5 Technical Requirements - http://support.apple.com/kb/SP517
Note that Leopard is very costly because of demand, and is now 4th generation system software. Apple typically only supports the last two generations, sometimes stretching to last 3. New software developers rarely support PPC Macs now. Many of us still run PPC Mac and old systems but it's like being given a 20 year old car, so don't have high expectations.
Texas Mac Man, Leopard disc sources - https://discussions.apple.com/thread/3636662
"a brody" post about upgrading to Leopard - https://discussions.apple.com/thread/3373795