Your Tiger iPhoto library may need upgrading with a Apple Library updater tool before you can use it with iPhoto '11. Reference. More iPhoto help just in case.
Make sure that that SD card is an SDHC card, and not just an SD card, or it may not be readable in your new iMac.
Here is the layout of the iTunes 11.0.1 Music folder on OS X 10.8.2. It may vary from what you have on Tiger.
Read the following articles before you move your iTunes library from Tiger.
iTunes for Mac: Moving your iTunes Media folder.
iTunes: What are the iTunes library files?
When you are satisfied that your iTunes content is moved over to Mountain Lion, you may wish to deauthorize the G5 from iTunes.