In all honesty though, this is something that swings back and forth with each series. The 4xx series by nVidia have been some of the hottest and loudest cards to date, maybe even worse than the current Ati lineup.
Then it also depends on what kind of specific manufacturer you choose. For example, some MSI cards come with after-market coolers which are much more efficient and since they don't need to run the fans at full tilt to cool the card, they are also considerably quieter too. An Ati 6950 with one of these coolers can very well be quieter than a similar nVidia card with a stock cooler and vice versa.
Lot's of variables, it's not always easy to choose