Nvidia pascal is likely to be revealed at Computex 2016 Taipei. Tesla P100 the first of all targeted to developers has enormous upgrade over previous generation K80 has 8.74 TFLOPS Half.Pr computational power, newer card almost three times powerful with 21.2 TFLOPS and almost double over M series.http://www.anandtech.com/show/10222/nvidia-announces-tesla-p100-accelerator-pascal-power-for-hpc
Same can be expected from GTX 1080 or GT X80 whatever they name it. http://wccftech.com/nvidia-geforce-gtx-1080-1070-computex-2016/
As VR is most demanding, the more computational power you have the best, also one can expect Maxwell to be more in purchasing range once fully blown pascal is out.
Its more sensible to wait to purchase system till mid June as any way Rift will not going to be shipped before that.
One must never buy supporting system of highly volatile market (Pc/Mobile) before you receive actual product (VR headset here) in your hand which is going to use its computational power. In same sense down the line per-order mostly sucks..Its mostly over enthusiasm that forces us to act otherwise. Whole pre order market survives on hype train..