My monitor for my gaming machine is not 1080p, I have 2 AMD 6950 video cards and I've never really been motivated to upgrade my monitor until I upgrade the whole system. (working on it)
I have to run the Rift in extended mode and I'm not sure if this has something to do with the inability for the it to mirror to my main display or not -- seeing as how my main display has a smaller resolution. If the demo/game has a "direct to rift" executable, then it will shoot directly to the rift but often I have to drag the window to the rift and alt-enter to full screen it on the rift. It's sort of weird.
Also, with my setup, is it possible to mirror what is seen on the rift to the display? It would just make demos that much more interesting to watch.