With the 5830 they can differ vastly due to ATI not handing them a reference card. They can come on 5850 boards, 5870 and so on. As long as it has a decent cooler? well the GPU and ram are all the same, so just get the cheapest one tbh.
As for the voltage? IIRC MSI afterburner can be sorted to use on any 5 series. So paying extra for that is madness.