My suggestion: Check the level of Linux support when it comes to drivers. Avoid manufacturers who put Linux support on the back burner.
As for the voltage question, some of the high-end video cards have some reasonably high power demands, and thus require a healthy power supply. In other words, don't install a video card that needs a 400W power supply into a box that is being run on a 250W unit. As for the specifics about the actual numbers your specific card might need, check with the manufacturer -- J.W.