Official haswell ram voltage is max 1.5v, but you can go higher if the mainboard allows it. As far as the ram frequency is concerned, I don't think that there is a limit, the memory itself maybe. But ram frequency is pretty pointless anyways since you cannot oc the non k 4770.
If you plan on purchasing the 4770, any 1333/1600 modules rated at 1.5v should work out of the box. Higher frequency modules may get downclocked by the BIOS at 1333 or 1600, but you should be able to manually adjust them from the BIOS at their marketed speeds.I don't think that paying a price premium for higher than 1600 is worth it if you plan on using them with a 4770.