Just FYI, there is no cost difference between 10.5 and 10.5.8. All "point" upgrades (upgrades which change the third number in the version number, like 10.4 to 10.4.8) are absolutely free, and should be installed.
If you install 10.5, run Software Update to download and install all pertinent software and OS upgrades to your system. After doing this until Software Update reports no more updates are available, you will be running 10.5.8.
Major point upgrades (like from 10.4 to 10.5) cost money, but "minor" point upgrades are free (and good for your soul as well).