It is doubtful you'll need to know any of this, but since you asked, here is my knowledge of it.
Most HDTV's these days have 4 different modes... 480p, 480i, 720p, and 1080i. (I'd be pretty surprised if you don't have 720p actually). They're just different ways of broadcasting a signal. Each broadcaster picks one of those resolutions, and broadcasts all of their HD material in that resolution.
When a picture is displayed, it's actually drawn many times per second, one line at a time, starting at the top and working it's way down. The "i" stands for interlaced, the "p" for progressive scan. Interlaced = alternately drawing the lines; odd numbered lines on it's first pass, then the even numbered lines on the next pass through (old style of display). Progressive scan = drawing all the lines in order on each pass through.
Traditional TV broadcasts in 480i. That is, 480 lines of resolution on the screen, drawn in an interlaced manner. This method of display was necessary in the past to create a picture that updated quickly enough on old TV's so as not to be too jerky.
Most HDTV these days is in either 1080i or 720p. ABC, FOX and ESPN broadcast in 720p, while NBC, CBS and most of the other HD channels broadcast in 1080i. There are pros and cons to each. Basically it comes down to 1080i having more lines on the screen (sharper image), but 720p being progressive scan (smoother motion). So it makes sense that stations that show more sports choose 720p, whereas movie channels usually go 1080i.
New formats are on the way too. 1080p may be next. The TV I have will display up to 1920p, but my HD Box only goes as high as 1080p.
In any event, if a signal is broadcast which is above what your TV is capable of displaying the TV or HD box will usually automatically downconvert it to the highest level your TV can show. So if for some reason you don't have 720p, ESPN will be downconverted to 480p. You'll still get a picture, but you'll lose a lot of the crispness. It wouldn't be a whole lot better than what you view now. I doubt this is the case though...your TV, unless it is pretty old, should be capable of showing 720p.
On many TV's you can force it into a particular mode and have it up or downconverted, but usually it's best to just set it to auto-detect the mode and let it figure it all out. Upconverting 480p to 1080i is pretty much pointless since the TV has to simulate the extra lines to compensate. Forcing it into a downconvert is also not a really good idea...you'd just be throwing away picture detail needlessly.
And that's all I have to say about that.
