For each, it depends on the form factor of the unit as well as the rest of the system specs. The integrated Intel HD 4600 should be more than enough for watching videos, but perhaps, where you're lacking is RAM or even the processor.
For both systems, the Micro Form Factor and for the 9020 USFF, there are no graphic expansion slots. For the remaining form factors, the desktop version has a 255 watt power supply and the Micro-Tower has a 290 watt power supply. This isn't enough for much of a graphics card upgrade at any rate. The best you could get would be little more than your current on-board graphics, but with it's own VRAM. Again, if you're short on RAM, perhaps that would be the better upgrade.
Just some food for thought.
-Wolf sends
Dell Optiplex 9020
Dell Optiplex 3020