I like using OCCT to test a power supply, it stresses both the CPU and GPU, taxing a power supply more than any single application you will use. OCCT also charts various measurements during the testing, but either it doesn't like 64-bit Vista, the particular sensors that my motherboards use for +12v measuring, or both. So I use both OCCT and my trusty multimeter.
The GTX 260 is a powerful video card that requires a lot of juice. The minimum power supply recommended by nVidia is 500 watts, though many people have found that a 500 watt PSU isn't enough when the GPU is maxed out. Personally I probably wouldn't use less than a 650 watt power supply with it.
At idle the OCZ Z Series 650 ran a steady 11.74v, well within the +/- 5% industry standard. Maybe once in the 30 or so minutes I watched it, it dipped to 11.73v.
I fired up OCCT and the meter dropped to 11.69. It fluttered between 11.69v and 11.66v during the entire hour test, never once dropping below 11.66v. I added the OCCT chart just because I had it, but it did not reflect the measurements made by the multimeter. The voltage only occasionally dropped below 11.69, maybe once every 10 seconds or so rather than a constant flutter depicted by the chart.