That would probably be better, but you have to factor in the cost of Digidesign DSP hardware. Last I checked, a simple mix core card is over $2000, plus you need an IO box for another few grand. A top of the line mac and a Digi 001 or 002 or MBox is cheaper than an old computer and digidesign DSP hardware. That is why us LE users really want dual processor support, we get double our current performance (roughly)
About the noise, I havent heard one in a quiet room, just at an apple store, and I didnt notice much, in a studio, the computer would be in an iso box anyway, even quiet computers. I see it as a moot point, as many high end studios either have iso boxes or machine rooms.