Hi Jules, Snow Leopard/10.6 would be cheaper than Leopard/10.5...
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)
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 is the only hangup...
http://sourceforge.net/projects/leopardassist/
List of Applications Not Compatible with Leopard...
http://guides.macrumors.com/List:Applications_Not_Compatible_with_Leopard
Snow Leopard/10.6.x Requirements...
General requirements
* Mac computer with an Intel processor
* 1GB of memory (I say 2GB at least)
* 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/
If it's a core Duo & not a Core2Duo, then it'll only run in 32 bit mode.