Computer will not post

I decided to upgrade my Alienware X51 R2 desktop.
I got an upgraded graphics card ( GTX 1080ti), the card wouldn't fit in the Alienware case. I was able to get a larger case from my friend, and an upgraded power supply to be able to power the graphics card, it's a thermaltake smart 700w PSU.
The computer still would not post, so I tried a new motherboard, I got a Prime Z270-A, with an i7-7700k cpu.
I tried using the stock ram from my old PC, I also tried 2 other sticks of RAM, also tried using just 1 stick of RAM in all 4 different slots, still nothing.
We reinstalled the bios, we also disconnected the bios battery for 5 minutes and reconnected.
I've tried 3 different monitors, I'm out of ideas on what to try.

Bruce said:

let's go back to a known, good point in time.
and since there will be some component swapping, let's remove the case from the equation and do all this on a piece of cardboard on a bench somewhere.

so - setup the original rig in the original configuration and try to at least get a PC that boots.
only use the core components to get the rig going, no GPU if you don't need it, only one memory stick, only one drive if you have two, no DVD unit, that sort of thing.

we just want a reliably booting rig for now.

when happy, than swap in the new GTX1080ti card and try that.
and go from there.

Click to expand...

So use the original motherboard & cpu, with the original RAM, and original power supply?
Then try to add 1 new part at a time, like process of elimination?