1. Click here.
2. See #1.
3. It depends on why you want to do it.
4. Keep 10.4 on a different partition or drive. The Disk Utility on the Mac OS X 10.5 install DVD can repartition the drive without erasing it.
For programming, Xcode is your best bet if you want to code for Mac.
Yes, the processors can be upgraded. http://www.everymac.com/systems/apple/powermac_g4/faq/power-mac-g4-how-to-upgrad e-processor-zif-agp-mdd.html