Yes, assuming you're tuning into an HD channel at a time of day when they're broadcasting actual HD programming (and not just upconverted SD - generally all non-reality primetime shows are in HD these days, and more shows at other times of the day are going HD slowly but surely).
Edit: Also assuming your video card's output resolution is greater than or equal to 720p in a 16:9 aspect ratio and you're watching in full screen and not a window...but that should go without saying
