Upgrading my Graphics Card, HDMI and Virtual Reality Headset

I have a Dell XPS 8700.  I want to upgrade my graphics card to a NVIDIA GTX 970 or 980 TI.  Will the GTX 970/80 TI be able to fit into my XPS 8700 Motherboard?  My second question is about the HDMI Video output. I've read that for some, when they connected their HDMI to their television, they were presented with a black screen. While I won't be connecting it to a television (I'll be utilizing it with HTC Vive Virtual Reality System), I wonder if I'll encounter the same problem when it connect my VR set to it. If I do, how do I go about correcting the problem?

Thank you for your help.