I know that 3D games have the advantage of being able to transfer graphics processing onto the graphics card. Does anyone know if 2D graphics can also be handled by the graphics card (hardware rather than software)?

How do 3D and 2D graphics compare in terms of processing power usage?

3d is always worse in terms of processing power. Complicated algorithums to decide what you can see round what, shadows of things, interactions between things, etc is complex, 2d you have flat things that are either in front of behind, and in terms of xna can be achieved by using levels for the drawing call so you draw in order from back to front the spritebatch does its "thing" and basically the framework ends up making 1 image which is sent to draw rather than tons of different bits.