Dell XPS 8900 and EVGA Nvidia Geforce GTX 970

GTX 960 is fine with stock power supply GTX970 needs Corsair CS750M  or EVGA SUPER NOVA NEX 750B1  P/N: 110-B1-0750-VR

TL;DR - I would upgrade the PSU. a top notch 550W like the EVGA supernova 550 G2 is more than enough, but since the price of the 550W, 650W and 750W are usually within a few dollars, I'd go for the 650W or the 750W

I put one in and it is working great.

I truly hope it continues to work great.

Several have reported GTX 970s are working "great" and later have reported back their systems were unstable and then had a PSU or a system failure. These reports were on the XPS 8700, however the 8700 and the 8900 are nearly identical as far as power. They have the same PSU, and the CPU and motherboards seem to have nearly the same power demands.

I think the issue is  several of the factory overclocked GTX 970 cards ( MSI, and Gigabyte gaming models) can pull more peak power than the stock PSU can deliver. This isn't a steady-state load, but a short spike, and I suspect if this happens it can cause instability.

A larger output and higher quality PSU can deliver this power cleanly and not degrade power to other parts. 

I run seasonic X-650s in the family's 8700s that are gaming systems and they run GTX 970s flawlessly.

I put one in and it is working great.  It fits without issue and the right power connectors are present and ready to use. You need to have a monitor with an HDMI connection to get it to boot up the first time.  I started with DVI and was dumbfounded for a couple hours trying to figure out where my BIOS post was!

Once you have made it to the Windows desktop and installed the updated drivers, you are good to go.  It works with any display now.  So yes, a GTX 970 and XPS 8900 (with i7-6700) will work well together.