I think the unit you want is Watts (W), not Volts (V). Volts will depend on where you are located; e.g. in many parts of the world (including the USA), line voltages are ~115V-120V, but in other parts they may be 230V-240V; a few places have other voltages, such as 100V. What varies based on the graphics card(s) is wattage needed. The PSU sizes recommended are for the entire system, with a single card.
A quick and dirty way to identify the absolute maximum amount of power a graphics card needs is to count the number and type of PCIe power connections it has.
A PCIe slot can provide a maximum of 75W. A six pin PCIe connector can provide 75W, and an eight pin connector can provide 150W. In the case of a card with a single PCIe six-pin connector, the most that card might draw is 150W (75W from the slot plus 75W from the single 6-pin connector). So, if the PSU recommendation for one card is 400W, adding a second one means you will need no more than 550W. This number is obviously a high estimate, as the card may need as little as 76W (just enough to need that connector), in which case a 500W PSU would easily be sufficient.
The best thing to do is use a PSU sizing calculator such as the one at
http://www.extreme.outervision.com/psucalculatorlite.jsp which has built-in tables of what graphics cards need, or find a review of the card in question and see if it states how much power it uses.