I just got the wd 750gb black sata 3 drive. This thing is loud as heck. What wd drive in the sata 3 range is quiet? It has to be 7200 rpm though. And under 750gb...
There are no 3.5" (desktop) 7200rpm drives which can be said really quiet, and the ones which are not loud have not any noticeable performance edge over 5400rpm ones.
You may short stroke some sub-6000rpm drive trying to achieve a decent trade off (I mean a 1Tb one such as the WD WD10EARS, the Hitachi 5K1000.B or 5K3000, or the Samsung F4)
If you want a sub 750Gb 7200rpm drive, I think that you should look at some 2.5" (notebook) model, such as the Seagate Momentus 7200.4 (but not the 750Gb), or the (somewhat less reliable) Momentus XT, or even the latest Hitachi.
Even a WD Scorpio Black can be enough quiet (so less quiet than the above quoted drives), but just when suspended and/or encapsulated, IME.
Unfortunately most of them aren't SATA3, AFAIK (but I don't see any need for SATA3, as any mechanical hard drive is just currently unable to saturate the SATA2 interface).
Last but not least, you may also give a look to a WD Velociraptor, but it's very expensive, and so I don't know if it's really preferable to a 256Gb SATA3 Crucial m4 SSD.