standard is 1.000V, in idle mode 0.95V , light OC versions have 1.025V...u can go up to 1.150V safely with OC without send ur card to the hell , some models get up to 1,200V, but just for extreme needed.... 1000Mhz GPU Clock u can reach with 1.130V, depend on model..( just as sidenote, since u not overclock)....
i use EVGA Precision for my card... u can use MSI Afterburner for change voltage, aswell
http://www.evga.com/precision/
http://event.msi.com/vga/afterburner/download.htm