I have 2 5870s, one is saphires vapor-x and the other is a HIS one (didnt plan on getting 2 hence why both different brands) the vapor-x runs at higher fan speeds than the HIS one but is definitely quieter, in terms of cooling the HIS one can possibly even reach lower temps just loud as hell at over 70% fan speed. I also wouldnt say 1 5870 is overkill, maybe at less than 1080p but still is only equiv of a 4870x2 which I had before my 5870s and that def needed more grunt to run high end games on max, even 2 5870s doesnt dominate games like metro 2033 at max as have dips in fps in intense scences.
EDIT - oh and I have had a few xfx gpus and all have been fine, its just their motherboards I wouldnt go near even if you paid me, RMA'd 2 of...