The 720p standard is a resolution of 1280x720, so obviously not great relative to a recent sub $150 computer monitor, most of which will be 1080p (1920x1080). If the Samsung truly has a native resolution of 1400x900 (that is, it has 1400 LCD elements across horizontally, and 900 vertically) then it should look ok. If the native resolution is something different, you should use that resolution even if it's not very high.
Another thing to look at is the TV's "overscan" settings. Broadcast TV has garbage on the top rows that is actually encoded closed captioning and other info (like show title, channel, etc.), and all TVs are preset to enlarge the image so that this extra junk doesn't appear on your display. With a PC attached, you want this turned off. From what I saw of a Samsung manual, you turn off Overscan by setting the "Picture Size" setting to "Just Scan", and is only available if you use HDMI to connect the PC to the display.
Another thing to try is to look for an option called "game mode" (though I'm not seeing this in your display's manual), which turns off image processing by the TV that may make broadcast TV look slightly better, but usually makes PC (and of course, gaming consoles) look horrible. If you don't have "game mode" try disabling the various "filters" and other features that are supposed to improve the picture. Before you get carried away though, make notes on the original settings or look for an option to return all settings to defaults.
Here's a nice article on overscan:
And here's one on Game Mode: