As far as I can tell the model number of the mother board is U7084 0U7084
Your board has a pcie x16 slot just buy a card and put it in that slot.
Another problem is your processor. It is very slow if you try to put a graphics card the CPU will bottleneck it. So have to go for a very slow card like a hd 5450 $30. Or hd 6570 $70
My advise is that don't waste any money on this because if you want to upgrade any thing you will also have to update a whole lot of other things too.
You can build a good system at $400-$600.