Electro-Magnetic Interference (EMI) doesn't really affect you -- I think you get more radiation exposure from the power lines near your house than from your computer. EMI affects the electronic components around the computer. If you have a radio near the system and it gets static or a TV that gets snow or those lines through the picture when the computer is on, you enable Spread Spectrum and that lessens the EMI.
In general, the less components on the board, the less power it will use. On a board with more features than you need, you can disable some things to save power. Mostly we're talking about a couple of watts here or there -- the Sandy Bridge CPUs are designed to be quite power efficient.
The power phase design of the mainboard doesn't affect efficiency. It supposedly affects overclocking -- more power phases equals smoother power going to the CPU which theoretically means higher overclocking. For Sandy Bridge systems, overclocking depends more on the CPU than on the power phases.