Can I install my only video card in any of the PCI-E x16 slots?

[Phusius]

The main PCI-E x16 slot, the very first one on motherboard, well the screws on the case won't hold it secure, lost it's threads I guess.

Can I install my graphics card in the middle of the motherboard, the second PCI-E x16 slot and still be 100% normal?

 

[cadaveca]

Yes, you can, but it will only get x8 or x4 link to CPU. bottom x4 slot is connected via chipset, the top and middle are direct-to-CPU.

 

[OnePostWonder]

If you put some plumber's tape or something similar on the threads of the screw, it should be tight enough to hold things in place so long as you aren't jolting the case a lot.

This is what I would suspect to work with the details given, though your situation may be a bit worse.

 

[Radeon101]

This happened to me before, all you need is a thicker screw to make up the difference in the hole size.

 

[Kreij]

Get a little tab of metal about 1/16" think and drill a hole in it that the screw will tap into tightly.
Put it on the opposite side of where the boards screw into, so the screw goes through the stripped hole and into the new tab. It will hold like brand new.

 

[ThE_MaD_ShOt]

Great Ideal Kreij. Also you could get a very thin screw and nut to use.

 

[Wrigleyvillain]

Back on my DFI X48 T2R could get 16x in either. Was nice to have options.

 

[babash*t]

Depends if your board has the 16lanes on the 2nd slot, or if your BIOS can allows setting the 2nd slot as ur primary.
but I recommended using the method by @Kriej

 

[Tatty_Two]

I'm lazy, I would just stick it in the 2nd slot (not Black x4), at x8 you would not really see any noticeable difference.