It looks like that's an AM2+ motherboard, so you can use AM2+ CPUs. It also has a pci-ex x16 slot for a videocard. That Sempron is a slow single core CPU, and you only have 2gb system ram. It has a 250W power supply.
So then what I would do is first add a low cost videocard which can accelerate your HD video playback. It needs to be a low power demanding card. I'm thinking something like a GT 710. This is for video playback ONLY, don't expect to game with a 710. Also, be sure that the program you use to playback video allows hardware acceleration and that the hardware acceleration is enabled in the settings.
edit: this step comes after adding the card- You can monitor CPU, via Task Manager/Performance, while playing back a video. If video playback is laggy, and CPU usage is 100% or close to it, then you need to move up to a dual core or better. You probably won't have to add ram, but 4gb instead of 2gb would smooth out a lot of your computer's sluggishness.