Upgrade to a Flat-Panel TV
You'll encounter plenty of these types of number-letter combinations: 1080p, 1080i, 720p, etc. Without getting bogged down in the details, the bottom line is 1080p is the epitome of high-definition TV—in other words, the most lines or pixels available.
However, there is an almost imperceptible difference between 1080p and 1080i. You will likely notice a different with 720p since the visual quality diminishes with the number.