Tegra2 is the new talk of the chipset town when it comes to HD 1080p decoding and output.
Pretty much any modern Nvidia GPU >= 9500GT will do 1080p without breaking a sweat.
The GT220 and up seem to be the best current models. I'm looking for a fanless one before I replace the 9500GT here with one, oddly enough to improve *analog* playback, not HDTV.