I see lots of speculation around about what graphics card you need for good VR experiences. At this point we are at least a few months away from availability of commercial VR systems for PCs. As such the advice I can give is pretty easy right now- wait if you can! If you absolutely positively need to buy a new graphics card now, go ahead. But if you can wait until you are actually receiving your VR headset things will be cheaper and there will be more information about upcoming cards, relative performance for VR scenarios and the needs of various titles needs.
In the end the answer won’t be straight-forward because it will all depend on content. Can you run VR with a HTC Vive with an older graphics card? Probably if all you want to do are more simple experiences. Heck, you could almost get the required 90FPS on an integrated GPU if your scene is just a simple cube (but maybe not quite). But the key thing is that you REALLY NEED to make 90fps. All the time. Techniques like async time warp are interesting but they have really visible artifacts when used with positional tracking and if you have tracked controllers they will be VERY noticeable (since every timewarp your controllers will glitch in their positions). Good experiences in VR are more about consistent smooth motion that 100% matches what you expect even more than realistic rendering. For example one of the demos we show is Job Simulator where everything is a cartoon representation of objects. It doesn’t look like reality but the Owlchemy guys did such a good job with it that it FEELS like reality.
So what am I going to do for my personal rig? First of all, wait as long as possible. Second, I’m not going to cut corners. Like most tech VR equipment will great much less expensive in the future. But this is the first generation of real products and its not going to be inexpensive for a quality setup. In the past I have never bought a GPU for much over $300, but I’m expecting to spend in the $650-range for either a 980ti/equivalent or maybe dual slightly smaller GPUs since stereo rendering can do such a great job on dual-GPU setups. I know its a lot, but then again 15 years ago I used to have to spend $4000 for a high-end PC and the whole system with an amazing GPU should be less than $2000 now. Until now the difference between that $300 GPU and something better was really hard to tell for most people, but with these VR experiences it becomes worth it.