Pretty much anything new these days will require 10.6 or higher, because nobody writes PPC software anymore and haven't for years.
If you have a Mac that's so old that it's running 10.5.8, it is either:
1. A very old Mac indeed that is using a G3/G4/G5 chip or
2. One of the very first Intel Macs, also quite old now
If it's an Intel Mac, upgrade it to Snow Leopard 10.6.8 by ordering the DVD from Apple for $20, then use Software Update to update your Apple software. You will want to do the optional install of Rosetta (part of the OS 10.6 upgrade) in order to keep your old PPC-based apps running. If you can, I would recommend upgrading your RAM to at minimum 2GB (more if possible), a very cheap upgrade that will make a huge difference.
Of course, even Snow Leopard is now all but off Apple's radar for support, so the longer-term solution is to start looking for a new(er) Mac and replacements/upgrades for any PowerPC-based software you are running. The writing is on the wall ...