I have a new out of case build.
asus m5a99x evo 2.0 board
msi 760 2gig gfx
4 gig corsair 1333 ram
8320 fx 3.5ghz cpu
On my mother board there are 3 pci e slots. My computer will not post and will give a boot device led on board error when I plug in my gfx card to pcie slot 1 or 2. Slots 1 is 16x slot 2 is 8 x and slot 3 is 4x. When I plug it into slot 3 (at 4x mode) it runs just fine.
So what is the problem? I have the latest bios version. All parts are new. I tested this gfx card in a lesser system and it ran just fine playing latest games.
I also tried another gfx card 8800gts in the pcie slot 1 (16x) and it ran and worked fine.
Since you have everything correctly plugged and still failing, just in case check what your mobo manual says about the led on board error it gives you.
Also, at this point it wouldn't be a bad idea to test your pc outside of the case, just to rule out any shorting (sometimes the video card could make too much pressure on the mobo causing it to make contact with the case).
Test it with only the CPU 1 memory stick and the GPU outside the case and see if you have a successful boot.