Always is a truely bet buying a GPU on its release, just after 3 months later nvidia released a decent driver update. Apart being overpriced on day 1.
Probably 870 will release only on september.
I own a Gigabyte 770, but getting a 780 windforce in june.
PS: 770 Windforce will run Crysis 3 on max settings at a average 35FPS/1080p and 77c ...GO FOR IT!