I have my 8800gts running to my 42' plasma, 5000+ BE with 2GB of RAM with Windows 7 64 bit.
I use vlc, no extra codecs, haven't run into a file format yet that won't play for me. my downloaded 720p stuff looks fantastic. The TV i run through media centre and no issues, there, uses the whole screen. So can't offer much help on your hd box.