Resoluton of the TV and the resolution of the video out... as well as the video resolution of what you want to see. S Video is the popular method, as there is likely an S-video out on you PC if you have a secondary video card...

There are also video cards that can be bought that output TV type signals... there are tons of options, just get out there and look around... VGA (the RGB port) is probably ok for most, but not if you want to watch HD stuff...