The solution is to return your GTX 970 board and not buy Dell computers.
I have exactly the same problem.
Dell XPS 8700 (true for other models too) Black screen on boot up no matter how many video drivers you remove or disable.
Has absolutely nothing to do with the Power supply, the 12 volts on the 8700 is actually well above spec for the 980 let alone the 970 and when you are running anything normal with these thaey ony take 10 or 20 watts.
The games are the ones that take the juice.
As a matter of fact, the board powers up fine and turns the fans completely off after 30 seconds or so, so PSU is NOT the problem.
The Big Problem is that Nvidia, Dell, EVGA and every 970/980 manufacturer know that the Dell won't work with this card, they just don't bother to mention it.
It has been an ongoing problem since day one with this board and from everything I can tell, while they are both claiming to be working on it, the reality is their engineers are playing the finger pointing game each telling the other it's their responsibility and in fact nothing has been getting done.
It does not take a month to solve this problem if you have some one other than the bathroom clean out guy working on it.
The Dells actually come with Nvidia multi GPU boards these days and to release this board without even a note of warning that it absolutely positively won't work with a Dell XPS is irresponsible in the extreme on the part of both Nvidia and Dell.
Dell should have at the very least sent an Email to all it's XPS customers telling them this board currently doesn't work in your computer.
There is also no guarantee at all that brand X motherboard in your home made computer is going to work with it either.
I have done this sort of upgrade to my previous dozen or so Dells with other Nvidia video cards and had no issue at all.
Suddenly having it not compatible with the premier manufacturers premier video card and not bothering to mention it is Completely unacceptable.
And leaving it to the purchasers to "guess" what might be compatible (at their expense) is beyond contemptible.
Not working together is one thing, knowing about it and not telling you about it is entirely another and is completely unacceptable.
My solution: tell everybody you can about this, maybe loss of sales will force a more responsible approach in the future.
Best Regards.