My computer (dual 2.0) idles at about 60. When its really running it gets up to about 71 before the fans kick in. I read somewhere that the "optimum" temp for th G5 is 56 degrees. So I would say there's nothing to be worried about.
My quad's processors are at ~50C at rest and go as high as 69C for CPU B core 2 when making all four cores work hard (which isn't easy to do - need really hefty photoshop filters for that). The quad has amazing cooling though with water cooling.