FWIW, I'm running a WD20EARS in a ReadyNAS Duo and it's fairly quiet - not inaudible, but definitely quieter than the 1TB 7200.12 that it also houses.
However, The Seagate 7200.12 has made quite a bit of "buzzing" noise (not sure how to better describe it, but it's pretty obnoxious) over the past few months and its reallocation sector count has increased, so it seems to be failing.
I have my eyes on either the WD20EZRX, WD20EARX or WDBAAY0020HNC-ERSN.
Since the drive will be used in a NAS, I guess performance isn't too important, so at this point I'm thinking about just getting what is cheaper? There doesn't seem to be any major differences between the different models.
Sorry about the thread-hijack, but this seemed like a relevent place to post, at any rate