My X1800XT (same fan) was not loud at all. During bootup, for a few seconds it was VERY loud, as they all are. Then once it posts, it quieted back down. In a case with good air flow, the card doesnt get loud at all. I did do some tests with it, and ati tool. Mine never went above 37% fan usage, even during load. 37% is pretty darn quiet. Manually raising it up to around 60%, was pretty loud, but it never went that high by itself. My GTX's in SLI were louder than the XT was, but not by that much.
In short, in a good air flow case, not very loud. In a case where the fan has to spin up high to be cool, I imagine it would get pretty loud. Or in the summer time if your room gets hotter.