The "Thermal Design Power" or TDP for the GTX1060 is 120W. (The 400W figure is an approximation for an entire system including a 1060
, but the TDP is specific for the card itself
.) My own rule of thumb is to allow 300W for the motherboard, RAM etc and then add the TDP of the desired GPU. I've never seen an instance of a standard PC with 4 memory modules that would exceed 300W, assuming you have a quality-built power supply. Once thing I've noticed about Dells over the years is they have well-built, quality PSUs - if it says 300W, then it means 300W.
(I can tell you for sure that the wattage required for power supplies are typically grossly exaggerated; this, in large part, because of the flood of cheap units that are over-rated to begin with...it is exceptionally uncommon for a PC with only one graphics card to actually use more than about 400-500W these days, and even in the days of the Nvidia 500- and 700- series monsters, the total power actually needed would be about 550W. Unless you had multiple cards or a GTX590/690, anything beyond maybe 600W is truly overkill - again, assuming a quality PSU that actually puts out what it's rated for, continuously and at a reasonable temperature of say 40c.
Cheap power supplies won't do that.)
Anyhow, back on the first page of this thread, I believe you indicated the Dell has a 460W PSU, so that you would easily have 160W (or more) to spare for the GPU.
Based on the above, I firmly believe you'd be fine with the 1060. You should check to make sure your Dell has a 6-pin PCIe power connector available; even if not, the 1060 will almost assuredly come with an adapter to be used with an LP4 type connector (often incorrectly called a "Molex" connector). The PCIe slot itself provides 75W of the 120 needed, so that cable will only have to provide another 45 watts (3.75 amps at 12vdc) - well within safe limits of the cables and adapter concerned.
The 1060's are great cards. I bought one for each of my two sons last Christmas....they actually do slightly better in synthetic benchmarks than my old GTX780 did, if the comparison is of any value. And if you get the 6G version of the 1060, that's twice the VRAM of my old 780, too. You should be very happy with it.
Hope this helps, and happy holidays