I found the MUGEN heatsink and I'll give a try at that celeron, although here in europe G470 is cheaper. I was more worried about power consumption than horsepower, but one core plus at same powerconsumprion is welcomed.
Nice that you have access to the mugen, us in the North america cant get Scythe products until they rework their distribution. Just to mention the only thing that i did find bad on the Mugen is their mounting mechanism, not that its bad, but hard to do as its bolted throught the bottom, a little hard to do it alone, not a big deal either, but much easier to do noctuas/thermarlight/prolimatech mounting brackets. On the CPU, honestly i never have used or even seen the G470, seems in the US its like OEM cpu, mostly for manufacturers of pcs, seems the price in the US is very close to what the 1610 is, but idk in europe. On sandy bridge most of the dual cores will have very similar consumption on idle as alll downclock the same, the difference was on loads as some had higher defualt clocks, same happens with ivy bridge, now speaking directly on G470, idk its a single core 2.0 ghz celeron.... could be it consumes less having one less core than the 1610, but my experience with i3 2100T was it idle really low, very much the same as the celerons and pentiums, but all were dual cores, i manage to idle around 14W headless. Here is up to you, if you can get the G470 cheap and you don't need the cpu pwoer that 1610 has, then go for it. The mugen should cool either at the lowest setting fine, on my HTPCMI (check sig) i have a mugen3 cooling an i3 2120 at 300rpm and even under Prime95 doesnt reach dangerous temps, load that will never happen on real world usage, so overall i expect that you will be able to run it at its minium settings and be fine with virtually silent CPU cooler, again the only thing that its harder to mount, once you do it its fine, very secure. One other thing about the fan, it can be droped very low, but will also depend on the motherboard and how it handles PWM fans, Asus should be fine, as they have 20% minimun on the CPU 4pin heathers, and fanxpert should be able to overwrite it and get it as low as it can, but im just saying this in case you go with another motherboard that can impact how the fan can be manage.
I like Fractal Cases. Since I'll only have 4 fans (2xcase + cpu + psu), will it be capable to silence my 6 noisy HDD's?
The best is to suspend them or to enclose them, the mounting on the R4 is on rubber grommets that will help some, but wont be as effective as the others mentions. Get quiet drives is another key thing, SPCR liked a lot the WD Reds 3tb, im about to mount 8x seagate 4tb on my server that i have read comments from fellow forum users that are decent drives, but no direct review form spcr.
And about the PSU, I'm on a budget, so I don't want to spend more than 100€ in it. What about a Seasonic S12II Bronze? Now I have an early version of this PSU and ABSOLUTE LOVE IT (175€ in 2006, ouch!).
The hdds will probably be the loudest components on the build, i believe the CPU cooler will be virtually inaudible below 500rpm, and the case fans can be brought down with the included fan controllers to 7v where they will be really quiet, you can still drop them 5v if you still can hear the case fans. The only remaining noise source will be the fan on the PSU. I have no experience with Seasonic S12II series, the only thing i can point you is a thread Hows the Seasonic S12II 520 Bronze ??
Personally i always spend lot on PSUs, specially looking for something quiet, PSU is an item that you don't replace that often, i usually just upgrade motherboard/cpu/gpu, etc, but PSU unless its starting to fail or get noisy, i leave it, so i like to invest as much as my budget allows on them.