Dell does measure the noise of their products -- then hides them in a corner no one can find... except if you look really hard w/ determination:
The PDF for this model is under XPS.
It's fairly quiet but not silent -- 24~26 dBA/1m at idle depending on exact distance / position of listener. This is w/ Q9550 with 4 GB memory & 1TB HD. Power draw at idle is 62W, which is pretty good considering the monitor is in there too.
The problem w/ the noise is that if it is not quiet enough for you, there's probably not much you can do unless you're a diehard modder. The iMacs are significantly quieter. Our review of the 24" iMac ( http://www.silentpcreview.com/iMac_24
) is now 2 years old, but it measured 20 dBA/1m in a live room -- and sounded very softly quiet under almost all loads. The Dell was measured in an anechoic chamber where the SPL tends to read a touch lower than in a live room, but it was at a closer distance (by about a foot), so the two measurements may be close enough to call it level (as in a level playground). The 4-6 dBA difference will be quite audible, because 20 dBA is at the edge of ambient in most environments, while 24-26 is above it in many.
I'm listening to another Dell right now, sitting about 2 feet away on the desktop -- this is a Dell Studio Hybrid, rated at 24 dBA (at listener's position as per ISO 7779 -- which is around 2'). It's mostly wind turbulence noise, fairly smooth. I can tell you this is plainly audible, even while I am typing. The iMac was not audible while typing and much lower in volume even when just sitting and viewing the screen. My own PC below the desk is always completely inaudible from where I sit. It probably measures around 15~17 dBA and has the benefit of the desk blocking the sounds, and the fans positioned in the back more than 3' from my ears.