"The AX800 Extreme was somewhat hurt by the heatsink the makes it so great. My inability to use it with my stock CPU cooler hampered it's performance. Furthermore, I did have some stability issues, the R430 core still puts out a decent amount of heat and it seemed to overwhelm the SilentCool technology, enough so that benchmarking wasn't always stable unless I had some active cooling. I didn't push the card hard in overclocking because of these reasons, I clocked it up to 410/420 core/mem.
The radiator temperatures never got hotter than 95 degrees while running Futuremark 2005 with the main body of the heatsink at 108. At idle the radiator was at 82 with the main heatsink at 90. With a heatsink that was able to suck the hot air from the radiator it's temp decreased down to 84/79 load/idle with the main heatsink going down to 97/88.
The active cooling really helps keep the heat manageable and aids in stability, I'd definitely recommend active cooling helping the large radiator if you are going to push the overclocking."
FULL STORY @ GOTAPEX (http://www.gotapex.com/reviews.php?rev=videocards/asusx800silentcool/index.html)