10.6 (Snow Leopard) cannot install or run on your PowerMac G5, but 10.5 certainly will.
Your only choice - if you need iTunes 10 - is to upgrade your system to Leopard (OS X 10.5.x)
You will find that Leopard is still quite expensive to purchase, as most places still sell that for more than when it was still available from Apple - $125 or more.
tip - Don't get sucked into buying a set of grey DVDs for Leopard, which will only install on a single Mac model. Get the black label DVD, which is the commercial OS X installer disk.
Serendipity is a lucky guess !