Q: Transfer files from G5 PPC with FW 800 to new iMac, no FW

Here is one such adepter.

http://store.apple.com/us/product/MD464ZM/A/apple-thunderbolt-to-firewire-adapte r

You can network the devices together via ethernet.  Share the entire hd.

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If you have a second Mac computer, you may want to try to copy over as many files you can. One way is to use target disk mode.

http://support.apple.com/kb/ht1661

http://docs.info.apple.com/article.html?path=Mac/10.6/en/8443.html

"Installing OS X 10.4 'Tiger' on DVD-Challenged Macs Using FireWire Target Disk Mode"

http://lowendmac.com/misc/06/0710.html

Here is a picture of  a firewire port:

http://upload.wikimedia.org/wikipedia/commons/thumb/b/b6/IEEE_1394_Firewire_PCI_ Expansion_Card_Digon3.jpg/800px-IEEE_1394_Firewire_PCI_Expansion_Card_Digon3.jpg

Also:

Migrating from PPC Macs to Intel Macs:

https://discussions.apple.com/docs/DOC-2295

http://support.apple.com/kb/HT4796