Although TheBlade is certainly correct - there's a much easier way to calculate assuming that the timbre of the sound is similar (i.e. the tone - whine or whoosh, etc..):
You will only hear/notice the highest SPL emitted in most instances... so even if you have 10 25db sound sources - if you have just 1 45db sound source - your system will appear to you to have a noise level of 45db. Because the other noise is so much lower it will get filtered out by your brain much more easily.
However, the bottom line is that if noise is most important to you (i.e. you've already stated that you won't be benching, the OC isn't as important, and you only want to play a couple games, etc.) then you would be far better off going with slightly lower-end, cheaper components - and watercooling everything
By way of example, my sig-rig has ~14 120mm fans in it, 2 GPUs, and a CPU that will run circles around a stock-clocked 3930K in everything but the most heavily multi-threaded apps. Yet it's measured sound levels are 32db from 12in away (and it's completely inaudible from my chair)... because all of those fans run <900RPM on 1080mm of radiator space...
Sure, for the same money I could be running something much more "impressive" than a 3770K on a Z77 board... but then I'd have to run something air cooled... and it would be much, much louder!
EDIT: Oh and if you want to know how you measure
the noise - you simply need one of these
(or if you have a smartphone/tablet with a good mic on it... there are free/cheap apps
that can do the same thing
).Edited by DiGiCiDAL - 7/7/13 at 5:29pm