Mac OS X Leopard was discontinued and Apple isn't selling it anymore.
You have a Mid 2007 iMac, a Mac that can be upgraded to OS X Mountain Lion, if you want. Mac OS X Leopard is an expensive Mac OS X version, so don't buy it. Instead, buy Mac OS X Snow Leopard > http://store.apple.com/us/product/MC573Z/A/mac-os-x-106-snow-leopard It's a newer version and it will work better with your iMac. Make a backup before upgrading