The highest Operating System you can have on your PowerMac is 10.5 and it is not available for download. It must be purchased secondhand such as from ebay or the like . It is no longer available from Apple.
There is a problem however and that was originally it sold for a high price (I think it was about $120) and so still continues to be rather pricey. In comparison the OS after that, being 10.6 was sold for $30 and is available now for only $20, but 10.6 would not install on your PowerMac because it is not an Intel based mac.
I'm not sure how much benefit you would get from installing 10.5 compared with staying with what you have. You may be better of buying a secondhand but more modern, Intel Mac and purchase 10.6 Snow Leopard to run on it.
As roam said the max OS is 10.5 and this can only be bought from eBay, craigslist etc. However, I think that you may even notice a slowdown with an early Intel Mac (such as 2006) if you are coming from a Power Mac G5.
What you have to remember is that the PowerPC is a completely different platform to Intel, different advantages and disadvantages.
Leopard is about £40 in the UK which isn't too expensive but is still more expensive than Snow Leopard, Lion etc. Strangely enough Tiger seems to be the most expensive at about £45-£50.