The GTS 250 doesn't have adjustable voltage (Volterra Vregs), so you can't go over (assuming) 1.2v on it, the card's voltages also vary though, some manufacturers produced lower voltages, some as high as 1.3 (ASUS I think), so the only real way is doing a Volt Mod with a Soldering Iron.
I saw somewhere, someone had a jumper on the back of their card, they read with a Multimeter the other point was producing 1.3v, so switched it to that from 1.2v setting, but it made no difference to the OC,
I was gonna try flashing my BIOS to the ASUS DK, altering naming info and clocks to my own defaults (aswell as adding 2D clocks), but decided I couldn't be bothered and it'd probably fail anyway.
MSI AB and EVGA Precision work with any cards for OCing AFAIK.
The Voltage adjustments, obviously depend on whether the card allows it.