What is reasonable noise level?

It all depends on your personal opinion. For me, I don't mind the higher noise fans, as long as they keep my components nice and cool. Other people design their computers with silence in mind, either because it will be in a semi-public environment (living room, den, etc.), or because they enjoy the challenge of ultra quiet performance. It's really down to what you can stand and what you want to get out of your system.

Still personal preference, but my vantec tornado at 56+db dosnt even bother me, even though you can hear it through the house.

i would say my limit is around 35db but it is all personal preference... if the fan made an annoying noise my limit would be like 10db so i couldnt even hear it. but i can tolerate the low hum

But it seems anything below 30db doesn't have big cfm

Usually, more noise means the fans are working harder to keep things cool, which in turn usually leads to better performance. Less noise means the fans aren't working quite as hard, and the performance may or may not be as good. You have to decide for yourself what kind of performance you want, and then what kind of noise you can tolerate. My only suggestion would be to try a fan for performance first, and if you like what you get out of it, see if it's too loud or if its O.K. for your ears. Then you can get either a) a higher performance fan if the noise is below your tolerance level, b) keep the fan because it's right where you want on both fronts, or c) get a lower noise fan.

So, Any good quality fans with below 30db?

If that's what you find acceptable as a noise level, then yes.