NEW YORK -- If you haven't yet sprung for a high-definition TV set, the week before the Super Bowl is a good time to make the upgrade. Sports look great in HD, and football in particular is easier to follow on a big, sharp screen. Here are some ideas to keep in mind if you decide to go high-def in tough economic times. 4 What size? A common rule of thumb is this: Take the distance you'll be sitting from the TV set, in feet. Multiply it by four to get the number of inches of screen diagonal you should get. 4 LCD or Plasma? LCDs have become the default choice for HDTV buyers. But don't rule out plasma sets. These can be cheaper for the same size of screen, and yield excellent image quality. However, plasma ones are thicker. and heavier and use more power than LCD screens. The most noticeable difference is that they have glossy glass surfaces, which can produce annoying reflections from windows and light sources, so placing them requires some care. Rear-projection sets are still around, accounting for a few percent of the market. Consider these if you want the largest possible screen and don't mind that they're not flat, wall-mountable slabs like LCDs and plasma. Image quality can be good, but they're less bright than flat panels and even dimmer when not viewed straight on. -- When you explore screens that are around 40 inches, you'll be confronted with another question: 720p or 1080p? This refers to the resolution of the screen, with 1080p being the sharpest. But chances are you won't notice the difference unless you sit fairly close or the screen is very big. A 720p set is fine for DVDs, which aren't high-definition. If you plan to get a Blu-ray player or to hook up an Xbox 360 or other video game console to the set, 1080p makes more sense. For LCD screens 40 inches or larger and all rear-projection sets, 1080p is now standard. -- There's another feature that can have a big effect on the image quality: a 120-hertz "refresh rate," coupled with image-processing technology variously known as "Motion Flow" and "Smooth Motion." What does this mumbo-jumbo mean? Well, TV is shot at 30 frames per second, and movies at 24 frames per second. This is fast enough to give us the impression of continuous motion, but not fast enough to make high-action or camera-panning scenes look smooth and steady. High-end TVs now compensate for this by computing extra frames and inserting them between the existing ones. Combined with a screen that refreshes the image 120 times per second, this means smoother football action. However, not everyone will notice it without a side-by-side comparison with another set, so it's not a feature that will give you many bragging rights. -- So how does the set hook up? You'll want to have as many HDMI (High-Definition Multimedia Interface) inputs as possible. HDMI cables provide the best connection between a TV set and a signal source like a set-top box, DVD player or game console. They're also simple to connect, with one HDMI cable replacing as many as five video and audio cables. Three inputs have become standard, but some cheaper sets scrimp. HDMI cables can even be used to connect some late-model computers to the set. You'll also find a standard antenna input at the back of the set, in case you get your TV signal through an antenna rather than from cable or satellite. If you do, you have another motivation to get a digital set right now. Most analog broadcasts are set to disappear on Feb. 17 (though there could be a delay until June). -- Lastly, once you have your set, don't forget to hook it up to a high-definition signal! Some people get an HD set and then think they're watching HD, when in fact they're watching blurry old standard-definition programming. The cable or satellite company can walk you through getting an HD signal, but if you're in a hurry to watch the Super Bowl or don't have pay TV, connect an antenna and tell the set to scan the airwaves. The local NBC station should be broadcasting in high definition. Outdoor antennas are best, but you can pick up a modern indoor antenna designed for digital broadcasts for $50.