How much voltage for HZ's?

you are at the point to stop. Rarely have I heard of ram actually running AT 2.4v
I can tell you right now, if you run your ram at 2.4v for a few days, they will fry.

I wouldn't go above 2.3V maximum. Personally I won't put mine above 2.2V. The performance boost isn't that large and thus it is not worth the risk in my opinion. I have had way to many D9's fail at 2.3V.

Yeah at 2.3 they may last a few months depending, 2.4 I doubt they will last more then a couple of months tops before you start really degrading them. I like to keep mine at 2.3 or lower (right now 2.15) with active cooling. Although for benching on occasion I will run higher. Its up to you though and different moduels have different tolerances.

The ballistics tend to not fair well with high volts for very long, HZ are similer but a little hihger I think. Cellshock I ran mine at 2.35 for months with no problems (with active cooling). But d9 chips regardless of pcb will degrade with higher volts (its been proven)