Your never going to get a good answer because noise is very subjective and changes drastically depending on many other factors like other fan noise, ambient room/house noise in general, room shape, etc... Personally my house is VERY quiet and even my P180 setup with 4870 @ 30% seems rather noisy. On the flip side I take it over to friends houses to game and even with everyone else using headphones (so no game noises) my pc sounds way quieter since the ambient house noise is so much greater.
I have never used a GTX260/280 but if the cooler is anything like that on the 8800GTS 512 (and it looks like it) then it is actually pretty quiet. I have mine running at 50% generally but when it was in my P180 I was able to get the entire system to be completly inaudible at ~30% fan speed. And I mean completly silent, the only noise in my room was the tiny pump coming from a small aquarium at the other end of the room. The temps where too high to game but even at 50% the fan is about the same noise as one of my yate-loon 120mm fans running at 7v so not bad at all.
It was definitly quieter than my 4870 fan but with a little tuning the 4870 stock fan is acceptable to me, though barely. Hopefully the 260 fan is something like my 8800GTS otherwise I just typed all that for nothing 😉