Here are the values we got when we placed a sonometer 5 cm in front of the card and shut off all the other fans during our tests.
Though quiet when idle, like other high end cards, the fan of the 9800 GX2 accelerates significantly when you play 3D games. The noise is then quite noticeable and annoying because of the sound the air makes as it crosses the fins; more so than the 8800 Ultra, which peaks at an acceptable level. On top of this, the 9800 GX2 is the only card that doesn't send air outside the case, which means it'll rapidly increase the temperature inside the case, thus accelerating the card's fan (as well as the processor's). Nvidia, you can do better!