I get what you are talking about, but you are talking about an optimal frame rate. Very few games today run at these rates on our 'normal' 2d monitors. Do the games really have to run at 60 FPS per eye for the Virtual Reality 'trick' to kick in? As far as I know games on a standard screen play and look okay from 25 frames and upwards.
Basically you need to render two images instead of one per pass, so at the worst we will need double the graphics power for the same resolution that we used to on our normal monitors.
Ive got a 760 GTX Im pretty happy with right now, another one in SLI mode might do the trick when the Oculus shows its ugly face. If it isnt, Im going to be happy to upgrade to whatever will be required. There is no reason to think that Nvidia and AMD wont be up to the challenge presented by doubling the power requirement.