No, we won't email you... we'll answer you here in the forum. That way other people with similar problems can find this thread and answer questions and/or solve problems on their own.
What you "should" get is subjective, and completely up to you. Tiger has certain system requirements, and so does Leopard. Does your machine meet or exceed those requirements? Are there any features that were added to Leopard that would make you want Leopard over Tiger?
What gets changed on your computer when installing Tiger or Leopard depends on what kind of install you want to do... there are three kinds of installs you can do:
1) Upgrade Install -- this install "upgrades" your system from 10.3 to whatever you're upgrading to. User accounts, programs, documents, everything will be preserved. This is the riskiest kind of install, since leaping from 10.3 to 10.5 may introduce incompatibilities. I recommend against this kind of install.
2) Archive & Install -- this install preserves most information, including programs, user accounts, documents and some settings. It installs a clean version of OS X on your system, archives the old system (which is non-recoverable -- you can't go back if you don't like the new system) and is generally more safe than an "Upgrade Install."
3) Clean Install -- this is my most preferred method, and nothing on your hard drive will be preserved. You will lose everything, but you can restore your files and programs from the backups that you regularly keep (you DO keep regular backups, right?). This installs OS X as clean as possible, but you will need to re-setup any user accounts, copy over documents from your backups, restore Address Book entires, and reinstall all your non-Apple, 3rd-party programs. This is the safest way to ensure a clean- and quick-running system. It is also the most destructive, since you lose everything and it takes time to restore everything back the way it was.
As for where to get Tiger and Leopard, there are many places: brand-new copies, Amazon, eBay, Craigslist, etc.
Be aware that if you purchase Tiger, you will need to locate a retail copy that is compatible with your G5. You can use the gray-colored "System Restore" CDs/DVDs that come with new Mac computers in the box, but be aware that they are system-specific, so you will need to locate gray-colored CDs that are specific to your particular G5 model.
Off the record, I vote you go full-blown Leopard with a Clean Install and restoration of your documents and programs from your backup(s).
