Noise is a relative issue. The ambient noise level in your room, more than the components can determine how loud your computer will seem. If you computer room is like a recording studio and has a 20dB or less then pretty much anything you build short of a fully passive system will seem loud. If your room is the typical 30dB or so when all is super quiet in the home then it takes a bit more to be loud.
I have had Windowed, None Windowed and even Open Air builds and unless you have a part with an issue I have found every system I have built to be nearly silent at idle and under load I could not hear them over my games.
From a scientific view point of absolute sound measurement the 350D will be louder as it does not have the noise cancelling material of the R4, however from the subject the difference will likely be minor at best.http://www.noisehelp.com/noise-level-chart.html