The 6800 series are hot-running cards, but are also known to tolerate high temps to a certain degree. Depending on the ambient temperature, my 6800GT
idles at 60-70C, with the fan always running at 100%. Safe, but on the hot end of safe. part of the issue is the thermally-challenged G4 case and lack of fan control - in a well-cooled case (like G5) it would probably idle in the 50s.
It's a flashed XFX 6800GS card soft-modded into a 6800GT. I've been running it since January in my Digital Audio G4 tower that also contains an OWC 1.4GHz (1.33GHz factory overclocked 7455) CPU. Despite the high temps, I've never had any artifacts, even that time it hit 85C. If you search PC forums you'll see that overclockers routinely take 6800s beyond 70C under load without issues - but it pays to keep an eye on temps. I do.
I don't have any experience with 7447 CPUs, but in my experience 7451 (my original 2x533MHz CPUs) and 7455 CPUs run much cooler than my GeForce (just by touching them - my 1.4GHz CPU's temp sensor doesn't work for some reason).
The 9800 is a hot card too, but I doubt it will bother the machine - anyway it should run cooler than my 6800GT!
I just bought a Radeon X850 PRO that I'm going to flash...I'm curious to see if it runs cooler in my G4...I like the GeForce 'cause it's dual DVI, but if the Radeon is much cooler I'll put up with VGA.