rubber gromets for HDD mounting will not cut it - you have to suspend them in elastic.
the "quiet" HDDs are actually very nice, you don't hear the motor at all. however, unless you suspend them, you will still hear the actuator during read/write (which bothers me more, since it's not a constant regular sound that can be ignored)
That's why I'm going all SSD. If I had a large enough case I could probably go all passive as well since everything short of the HD7950/70 and GTX680 have passive coolers available.