Why can't I stream/edit High Definition video?

My computer has a 512MB ATi Radeon 3850 AGP graphics card, 2GB of RAM, and a 2.39GHz processor; I recently had a 64MB video card, and it has the SAME exact performance as the 512MB one. I have a pretty good internet connection, 54.0Mbps. Why can't I watch HD or edit in HD with no hle whatsoever? Thanks!