Hi, you're out of luck for 10.5 without an Instaal Disc.
Far cheaper to get 10.6.x
Leopard requirements/10.5.x...
* Mac computer with an Intel, PowerPC G5, or PowerPC G4 (867MHz or faster) processor
minimum system requirements
* 512MB of memory (I say 1.5GB for PPC at least, 2-3GB minimum for IntelMacs)
* DVD drive for installation
* 9GB of available disk space (I say 30GB at least)
Classic/OS9 Apps no longer supported.
Trouble is Apple no longer sells it, check eBay & such for the Retail version, not the Gray Discs...
http://www.ebay.com/sch/i.html?_nkw=mac+os+x+leopard+retail+10.5
You have to call Apple & likely ask for a Product Specialist to get it, if they still have it! Helps to tell them you have an iPad/iPhone & you can't run 10.6.
There are workarounds if the 867MHz CPU is the only hangup...
http://sourceforge.net/projects/leopardassist/
http://lowendmac.com/osx/leopard/unsupported.html
List of Applications Not Compatible with Leopard...
http://guides.macrumors.com/List:Applications_Not_Compatible_with_Leopard
First you need to research all the problems people are having with the higher OSX versions, & make sure you have a bootable clone of what you have just in case.
then you must get 10.6 if they still have it, install it & update to 10.6.8 so you have the App Store to buy & download the huge 10.8 Installer.
Snow Leopard/10.6.x Requirements...
General requirements
* Mac computer with an Intel processor
* 1GB of memory (I say 4GB at least, more if you can afford it)
* 5GB of available disk space (I say 30GB at least)
* DVD drive for installation
* Some features require a compatible Internet service provider; fees may apply.
* Some features require Apple’s MobileMe service; fees and terms apply.
Which apps work with Mac OS X 10.6?...
http://snowleopard.wikidot.com/
It's been pulled from the online store & Apple Stores, so you have to call Apple to buy it, last I heard.
Buy Snow Leopard > http://store.apple.com/us/product/MC573/mac-os-x-106-snow-leopard
Call Apple Sales...in the US: 1-800-MY-APPLE. Or Support... 1-800-275-2273
Other countries...
http://support.apple.com/kb/HE57
It looks like they might still have it...
http://store.apple.com/us/product/MC573Z/A?fnode=MTY1NDAzOA
If it's a core Duo & not a Core2Duo, then it'll only run in 32 bit mode.