The demo shown at E3 ran under DirectX 9. However, the video card(s) used weren't mentioned nor where the frames per second or other performance characteristics. My guess is Carmack will push the boundaries of performance in other areas as well (as he has done in the past) while making the game compatible with even older cards at lower settings. Remember Doom 3 had an advanced hardware detection algorithm that determined the most suitable graphics settings for your hardware. My guess is they will reuse that code for Quake Wars. Most likely, Quake Wars will become the new standard in graphics performance for gaming computers
Mega Texturing should be supported by standard texture support within the card. The only difference in Mega Texturing is in the coding. Looks like a fairly simple concept taken from the real world. This is how painters and other artists make their art. Ie, lay down a basic texture and layer more detailed textures on top of the basic textures. The combination of layers makes for a extremely wide ranging look without forcing the video hardware to load a unique texture for each unique look. Ie 12 textures allows 12 factorial combinations of unique textures assuming no limit to the depth of layers. So, the hardware only has to load 12 textures instead of 12 factorial.