3-prong plugs (which computers use) have never been possible to insert upside down, and 2-prong plugs have been required to be polarized (one blade longer than the other, so impossible to insert upside down) since 1962. Aside from this, all voltage rails in a computer are electrically isolated from both sides of the mains. Most connect their 0V rail to the ground pin of the wall outlet, but there is no path from the AC lines to the DC. Touching a PC case has a 0% chance of electrocuting you.

Regarding the graphics card, one side is covered by the fan shroud and the other by the backplate. No part of the bare PCB is exposed to the chassis. Even if it was, there is more than enough clearance to keep anything from shorting out, and if there isn't, a piece of foam is all it takes to fix that.