There's a huge amount of debate about this, so I wouldn't listen to any single opinion, or worry about it too much anyway. In general, the more pixels the better, and the more likely you are to get a smoother, sharper picture, other things being equal.
However, the correlation is a loose one, and other qualities are probably more important. The fundamental thing is whether a TV is genuinely "HD ready" or not. (HDTV ready or other descriptions do not count at all). That will give you at least 1024x720 resolution, which is sufficient.
Many here would prefer 1280x720 because it will match many of the likely HD sources, and should avoid the display having to downscale the video. However, I believe there are no 1280x720 plasmas, and few LCDs. Many HD displays now have 768 lines and work fine, but I wouldn't buy one instead of a 720 line display JUST BECAUSE it has more lines. Other things are probably more important.
Nick
Edit: You may find it easier to play films with a PC if you have a 16:9 resolution like 1366x768 or 1280x720, rather than 1024x(anything).