I just built my own gaming rig and debated the same issue. I wanted an NVidia card because I plan to use it later on for professional applications that will likely use cuda, but right now just wanted a great gaming rig. In the end I decided to go for a single EVGA Superclocked ACX GTX 770 which I got on NCIXUS for $309 brand new. Now I see that this same graphics card has gone on sale for $299. I honestly would recommend that you pick up the 770, as the 870 could come out as early as October, but as late as 1st quarter of 2015. I've gotten great performance from my 770 and may add another around Christmas time when I assume there will be some pretty good sales. That's just my two cents, but I hope it is helpful!