I would need to know the exact model Power Mac G5 that you own. There is a difference in the way modules can be inserted in the early 2005 model and the late 2005 model. Also, earlier than 2005 Power Mac G5 machines have less capacity for memory. Perhaps this site can help you decide.
The only instruction seems to be that you need to install ram in pairs. So it would seem that 4 x 256meg and 4 x 1gig for a total of 5 gig would be ok.
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