I have an BBA X800XL @ home, and I find it pretty loud. The normal speed isn't too bad, but during games it will sometimes ramp up to high speed (stock fan is thermally adjustable) and then it is very loud.
At work I have a Quadro 3400, which I believe is equivalent to a 6800GT (and right before that I had a 6600GT as a test card). Both the Quadro and the 6600GT seem quieter than the X800XL, and even when the 6600GT fan sped up, it wasn't by very much. Not sure how taxing the cad work is though, compared to gaming (CAD is very light on texture manipulation, so it's possible that it wasn't stressing the card much)
Also, I had an ATI FireGL 3200 and 7200 at work, and the 3200 was the quietest of all the cards, while the 7200 was on par with the Quadro, and the 6600GT was loudest (but only by a very small margin). We ended up going with NV Quadro cards because of driver problems with ATI, so noise wansn't really a large factor, but it is something I am concerned about at home. Overall I think the ATI drivers are good for home use, but even their FireGL drivers leave something to be desired in the CAD arena (Pro/E specifically, which is not that robust a program in its own right).
Anyhow, the main point is that from what I can tell, the cards have similar noise levels, and the X800XL is by no means quiet (but I may notice the sound more than most).
-D'oh!