Can my computer play games good and play HD videos good?

Recently I went and got a computer built from this guy my mom knew. I expected him to know things about video game graphics and such, but I was sadly disappointed. My computer seems to be a good computer from the specs, but it feels like there's a problem with something inside the computer. My monitor and graphics card are really good as far as gaming goes. What I'm not too sure about is the processor. I don't feel like this processor is good enough for my computer and it confuses me when no one gives me a direct answer about any of it. I notice when I play games that the frames are a bit low and from having this computer for about three weeks now I notice when I first got the computer the frames were fine. As I played it more the computer got slower to me. As far as videos are concerned I've never been able to truly play them in HD. I don't quite get why when I play a video and set it in HD it doesn't display in HD. Not sure what the issue is, but I'd love your help I'll list all my specs below this.

Graphics: ASUS Nvdia Geforce GTX 660

Ram: 8gb

Processor: AMD A8-3870 APU with Radeon(tm) HD Graphics 2.99GHz

Motherboard: msi A7MA-G55

Monitor: 1600 x 900 60hz

Power Supply: 550W

Hard Drive: 1T

Internet Speed: 8063 Kbps / 1147 Kbps