Re: Viewing a 1080i video file
There are so many things that can influence performance of HD content. First of all on Linux the GPU doesn't help that much, there's only Xv support for scaling (though it uses simple texturing on modern cards like on your 6600gt) and there's XvMC but that's only for Mpeg2.
All decoding of your HDTV content if it is not Mpeg2 which the CPU has to do all the work. This is a big task. The more optimized the codecs of the media player, the better the performance (dual-core would also help a lot) On windows the gpu can assist with decoding mpeg4/h264 and other codecs but this is not offered on Linux at the moment.
Overall I don't think your videocard is a problem but more the not so great linux support for HD-content.