A good GTX800 card might not appear until Q1 2015. While DX12 hardware support is uncertain, it's unlikely to make much impact for a long time since Game Developers won't be making DX12-only games, or even adding much code to benefit only DX12 when few people would have these cards.
I suspect the GTX700 series will work with DX12 mostly and the features it doesn't support won't even be implemented for close to 2 years, though again the GTX700 is still going to work just fine for many years.
Not sure of your budget but I like the Asus GTX770 (about $320).
I believe current AMD and Nvidia cards will be supporting DX12 anyway. It could be a long wait before DX12 games come out (mid/late 2015) and Nvidia 800 high end GPUs will be at a premium price. Just buy the best value performance GPU now and enjoy it.