you should first identify the video codec of your movies: personally, I got remarkable benefits with MPEG-2 and H.264 while I'm still working on VC-1. Videos encoded in this format have frequent issues with hardware decoding.
Anyway, you can check MediaPlayer Classic when playing any video: if you see "Playing [DXVA]" in the status bar, then the GMA is used for decoding and you should experience low CPU usage. Otherwise, something is not working.
There many guides on the Web that show how to ensure DXVA (which is DirectX Video Acceleration, BTW) works properly, but they may be confusing at first. Instead, I would suggest you try the following:
1) Reinstall the lastest version of the K-Lite Codec Pack. During setup (Advanced Mode), there should be screen asking you whether to enable DXVA or not. Check at least the first option (for H.264).
If it still doesn't work:
2) Open MPC, then File->Options. Select the "Internal Filters" item on the left and UNcheck all checkboxes on the right with DXVA in their name. Then select "External Filters" (on the left) and add the following filters (with the proper button):
Microsoft DTV-DVD Video Decoder
Microsoft DTV-DVD Audio Decoder
Make sure these ones are on the top of the list (if any other item appers) and select the "Prefer" option for both them. Then save and restart MPC.
This worked fine for me, at least for most videos.
Hope this helps.
Since you're running on a x64 version of Windows, you can also try MPC x64. It performs slightly better on my machine, which has a similar configuration as yours.s