An eMac with a CPU clock of 1 GHz or greater can be upgraded to Leopard. You will need to purchase a retail Leopard installation disc which will be a challenge to obtain at a reasonable price.
iOS devices running iOS 6 requires iTunes 10.6.3 or later. iTunes 10.6.3 is the last version of iTunes to support Mac computers with Mac OS X 10.5.8 with either Intel or PowerPC processors.
iTunes 10.6.3 for Mac (Leopard or later):MacBooks iMacs iPads AirPorts, OS X Mountain Lion, 28 years Apple!