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Here is Adobe's Leopard Compatibility PDF:
http://www.adobe.com/support/products/pdfs/leopardsupport.pdf
Basically it says that Photoshop CS 2 and earlier "likely to encounter issues for which there is no resolution". So yes, CS 3 is a prerequisite for Tiger. Since you have a PowerMac G5, you can install a second hard drive inside of it with Tiger or earlier installed to run CS2 or earlier.
Message was edited by: a brody