You could try updating the drivers from nvidia's website. If your video card isn't supported, you could try: http://www.hardwareheaven.com/nvmodtool.php
Then try playing the video on your laptop only. The 7150 might be able to play it, but it would probably choke once in a while. Also double check, if your cpu usage is at 50% while playing video, then it would really indicate that the video card isn't doing all the decoding work.
The 8400M might be better, but it doesn't seem to have hardware h264 decoding (which may impact performance). The 8400M GT however does support h264 decoding on hardware, which would pretty much guarantee that it could play hd videos.