If I remember rightly, toms hardware recently did an article on DX 12 and I believe it said a representative from AMD was present to endorse DX 12. As to whether your specific card will be supported, I think its better to wait until it actually is released before you make any further decisions. DX 12 may yet be put back from release dates, they may claim to support some but later can't because of XYZ... its really early days on this. Wait for release, see specifically what the situation is then and go forward on the basis of fact, not conjecture.
from what i know so far all GCN gpu will support DX12 (7k sereis and R series). for nvidia card that will be Fermi and onwards (400,500,700 series). but i also heard that there will be new feature that need new hardware that will only available on future gpu. so what current gpu able to support was DX12 feature that doesn't need new hardware (like cpu overhead optimization)