Your G4 mini is compatible with 10.5
Make sure that the install disks that you get are black, and not grey. No grey disk with Leopard will install on your mini, as they are specific for Mac models, and no G4 mini ever came with 10.5 originally. The black disks are the commercial version, which will install on any supported Mac, and your G4 mini is supported. You don't need to worry about that, as long as you purchase a black installer DVD.
You will need to upgrade to the maximum RAM memory. There's just one memory slot, which can hold up to 1GB of PC3200 SDRAM.
Bottom line - your mini is compatible with 10.5
The product code on the disk is meaningless to you if the disk is grey. No grey disk will work in your mini. You must make sure that the disk you buy is black.