Well, I'm running 2 770 4gb in sli. And while my Geforce experience says i'm not ready for VR because of my GPU, I can play current DK2 games just fine on it. I understand that the performance improving "don't bother to render what will not be visible once the screen has been squashed to fit in VR" technology will not be supported in the 770. It's only in the newer architecture 970 and beyond. So despite the 770's theoretically benchmarking close to the 980 ti (within about 10%); it's possible that there will be a signifiant performance hit without this tech. I.e. there *is* good reason to upgrade.
However, it seems that the latest 770 drivers are VR SLI ready from their blurb, allowing each card to render each eye, or take alternate frames. Which kind of adds to the confusion!
Obviously the consumer rift, is going to require lots more graphical grunt than the humble DK2. Due to higher 90Hz framerate and number of pixels to push. But I heard that it may also support 75 hz mode, so it may not be as bad as all that! I've survived on 75 Hz up till now.
Personally I was considering upgrading, to the 980 ti, but having looked at it, I think I'll wait until april when the oculus actually ships, see what it's like, because by then the 980 ti's will probably have dropped in price anyway and it may be that the 2 770s will be able to push enough pixels.