I have not gone for a HD TV yet so am probably not the best person to anwer....but...
HD TVs can display a number of lines, either 720 or 1080.
They can also display the lines "interlaced" or "progressive scan" (which is the best of the two)
So if you see a TV that is 720p it is 720 lines progressive scan.
If you see a TV that is 1080i it is 1080 lines interlaced.
If you see a TV that is 1080p then it is 1080 lines, progressive scan.
The best quality is 1080p, and in the long term that is what the TV industry is aiming for with HD.
BUT nobody is putting out TV at 1080p yet, nor will for a while. And 1080p TVs are still very expensive.
BlueRay and HD DVDs are put out at 1080p, but if your TV only shows 1080i or 720p then you will not see it at its best.
I think HD TV is in a hybrid state at the moment, working towards 1080p, but no really there yet, with most TVs sold being only 720p or 1080i.
Everyone who has bought a 720p or 1080i TV will probably buy another one in a few years that does 1080p, so they can get the best quality.
If you can, buy a TV that does 1080p (assumning you can afford it).
Note that buying a HD TV does not automatically give you HD TV, you need to subscribe to the Sky HD service (more money), of watch BluRay or HD DVD movies (but need to buy a BluRay or HD DVD player).