If we use dual monitors, does it consume more graphic card's power?
In other words: if we use a single monitor, will the graphic processing performance be better than dual one?
Yes, but if they are not both displaying 3D graphics, the effect on the GPU will be minimal. The second screen would also use up video memory to render the screen, but this will have a minimal effect as well, unless it was displaying 3D graphics with lots of textures.
Basically, you only have to worry about the available GPU power if you intend to play 3D games with two screens at the same time, or use powerful 3D/CAD software (but I cannot see how you could use both screens at the same time doing that).
You may also have performance issues if you have more than one very high resolution screen connected to very low powered onboard graphics.
Depends on the setup in question. Some setups run twice or three times the resolution and split it to multiple monitors(like the triplehead2go). Others have multiple GPUs that independently handle multiple ports on the card. If it's a DVI to DSUB dongle I don't think it has much impact-if any at all.
Yes, NVidia cards are known to be stuck in 3D high-performance mode when using dual monitors. It is apparently a hardware limitation, which can be worked around by forcing the card into power savings mode. See here http://httputility.com/article/dual-monitor-power-saving-nvidia