6600GT is more than enuf for 1080p HD videos. Everyone usually decodes HD in software mode using CPU, even if they hav a powerful GPU. Install a gud HD codec and configure it to use ur grfx card. According to me, installing K-Lite is the best, it's free and uses less system resource.
* If u don't hav K-Lite codec, download it and install from
here. Choose standard, basic or full pack according to you preference.
* After installing K-Lite codec, from start menu select ffdvideoshow video decoder under configuration sub menu under K-Lite Codec Pack. In the ffdvideo configuration window set the decoder to
Disabled for
H.264/AVC.
* Now open Media Player Classic and open its Options by pressing key 'O'.
Make the following changes as shown below for the Playback>Output :
Now either u can use Media Player Classic or Windows Media Player for playing 1080p videos. There are many more options that u can optimize for GPU decoding, you will learn it by urself if u can spend some time around the configuration options.