The hardware requirements for 10.5 are
- Mac computer with an Intel, PowerPC G5, or PowerPC G4 (867MHz or faster) processor
- 512MB of memory
- DVD drive for installation
- 9GB of available disk space
You should be able to install 10.5 on that Mac. You will need to find a Leopard install DVD which Apple no longer sells.
Someone posted as recently as Jan 20 that they were able to get Leopard from Apple by calling the phone-driven store at 1-800-MY-APPLE. Telling them someone gave you an iDevice seems to help them find it.
You don't say where you are. As far as I know, the call-in option only works in N. America. Like the other Allan says, they may not have it but it's a free phone call and, it they have it, it will be much cheaper than most third-party sources. Older Apple full retail install diks are in high demand and many sources really gouge you on the price, often charging more than a PowerPC Mac is worth in some cases