I've had my 8800gtx for a week now and can comment on it's noise levels in my P180 rig (all Nexus 120mm fans, Seasonic 650w, Scythe Infinity).
The card is silent when not under load, ie in Windows etc. For standard games that don't place too much of a strain on the card, it's inaudible.
The only time the fans speeds up is in games like Oblivion that stress the card.. that said, you need to turn HDR on and run in ultra-high resolutions to get the fan to rev up.
At it's loudest it's not an annoying sound, more like a whooshing of air.
Overall, it's NOT for a 100% silent rig, but the amazing performance from the 8800gtx series is worth a bit of pain. These cards will be very popular amongst the perofrmance/enthusiast crowd, so I expect to see some 3rd party cooling solutions pretty soon anyway.