480i is regular TV. 480p is the same resolution as 480i, but it is in progressive scan, which makes it look better. A lot of Xbox games are 480p (See back of game package.) 480p is also what progresive scan DVD players will play your DVD's in. Its not really HD though. 720p is higher resolution and progressive scan. This is HD. Most HD brodcasts are in 720P. 1080i is also HD and has higher resolution that 720p, but is not progressive scan. Niether 720p or 1080i is really better than the other one. There is also 1080p, which is the holgrail of HD, and is only aviable to the rich and famous ($$$).
Your TV supports 480i, 480p, and it may support 720p and 1080i or one of them. Som your TV may have 720p and not 1080i, or 1080i and not 720p, or it could have both. You can still watch HD it the one you don't have, your TV will convert the signal to the one that you have. You don't have 1080p. THose TV's cost over 10 grand