For some serious $, there are real good 350W fanless PSU's
here
That will only cut your PSU fans though
As for cutting dust, positive case pressure
does work. Dust
won't come in the crevices because there's air blowing out of them. That is the whole point of positive pressure - the effects of "leaks" are negated. You must, of course, filter
all your intakes well. I just take out the filters when they clog up and wash them with soap and water, put them back when dry, about half an hour.
you'll probably have to mod your case to add intake fan(s) to get more air in than out. A rheobus is pretty crucial too. You don't want 90 psi out the cracks or anything, the positive pressure is very slight. I adjusted by holding a burning incense stick by a drive bay crevice and tweaking the fans with the rheobus until the smoke was barely blown away from the crack instead of being sucked in.
Yes, the filters cut down the flow versus not filtering. This in no way means you'll necessarily choke your case for air. You just have to plan things. There's different filters too - higher flow ones that don't filter as fine, and really fine ones that cut the flow more. Your typical case fan filters don't dramatically mess up your flow. Flow increases with noise and dust Everything is a tradeoff. HTH
Peace
Sanaka