Yes; buy a Mac OS X 10.5 DVD from a source such as Amazon or eBay. Additional steps are needed if its CPU is slower than 867MHz.
Depends. If it has an 867 MHz or faster processor then you can upgrade to 10.5.8. You will need to look for a Leopard retail installer set on eBay or Amazon. They are no longer sold by Apple. Be sure you only purchase a retail set of discs. Do not buy OEM gray labeled products or so-called "universal" installers.