I have also two HDD inside(160GB+300GB). But SSD is more comfortable so I choose an Intel 320 SSD(160GB).
First, I remove 300GB HDD from under bay, and set new SSD.
Then I use "Carbon Copy Cloner" to clone 160GB HDD.
And remove 160GB HDD, then 300GB HDD install on upper bay.
Now I use 160GB SSD for Mac OS, and 300GB HDD for my documents.
Buy a Hard disc case and 160GB HDD now use atached HDD.
It is a very simple and cheap way to use SSD.
As you know, G5 has a SATA-1 interface. Intel 320 SSD has a SATA-2 interface but it works completely.
Good luck and God bless you.