They wouldn't be selling the product if its weight alone would damage your motherboard.
Since the video card should be screwed/clipped onto the back of your case (where your VGA/DVI cable connects) that should be supporting a bit of its weight. So your PCI-E slot is not supporting 100% of its weight anyhow.
But as stated above. Don't be tossing your system around like a sack of potatoes. Treat it like it's full of expensive glass vases, and it should be fine.