The more you can get the better, of course, but if you want a smooth experience on DK2, the frames should be around 75 (don´t scare just yet) and in the consumer version should be aorund 90fps. That´s a lot, indeed, but Oculus created a technology called Timewarp. What is it? I quote from flyinside kickstarter campaign "If you look to the left, but the last picture the Rift has is not quite in the right place, it will stretch and move the image to match. Even if the software falls behind, the illusion is maintained and the virtual world feels real! If FSX stutters a little bit, or it misses a frame, FlyInside picks up the slack. It uses timewarp in a parallel manner, sometimes multiple times in a row, to fill in the gaps while FSX is busy. You can turn up your graphical settings, and even while FSX is stuck loading a new chunk of landscape or buildings, FlyInside will keep your view smooth and immersive."
It´s witchcraft, believe me xD I`ve got an i7 with a 780 and Asynchronous Timewarp gives me a constant 75fps and with some carenado planes (I don´t have PMDG 737. Hard to get 75 with it, probably). The truth is, that since you use oculus DK2 with its graphics limitations, you don´t need all the graphic sliders to the right. Flyinside takes care of it for you (if you want of course), and personally I don´t use auto-gen since I fly with photoreal scenery and that´s a big hit on frames. The immersion is so overwhelming that you don´t really miss those nice HD graphics . The only problem is reading the gauges properly. You have to get close to them phisically, but that will be solved with the consumer version of the googles.
But with the consumer version, and its improved graphics,I`m afraid we´ll have to get a 980ti to fully enjoy it (I might be wrong tho)