1. From Mac OS X Leopard 10.5.2-AMD-EFI-Rev2, how do I upgrade to 10.5.3?

2. If I use Apple's built-in Software Update to update firmware, applications, or even a whole upgrade, will my system break?

To 2: You can use Apple's Software Update to update software like iTunes oder QuickTime, but for system updates you need patched files - like the AMD Software Update does.

-crawle

So there's an built-in-AMD-Software-Update program in 10.5.2-AMD-EFI-Rev2?

No, there's no built in updater 'yet'. Go here to catch up with AMD SWU.

Thanks. :P

So what is the best way for one who has a Leo4AllV3 install to upgrade?

I would update your kernel to the new Voodoo kernel before starting...
Use AMD Patcher to patch single updates that you can download right from Apple's Download site. Then before updating (in order of course) you need to make backups of your kernel extensions and also ensure you have the proper kexts and other files necessary to update (check the update threads in this forum for help and a listing of these files). Then run each and make sure you reboot a few times to ensure each update works and don't worry about your sound or video not being exactly the way you had it set up until you are fully updated.