Couple of tidbits I've picked up.
SD displayed on some HD sets looks worse than SD on a lot of SD sets. Many manufacturers dont spend a lot of money on downconversion hardware, and they dont have to since the showrooms show the finest HD signals over HD cabling to HD sets in HD mode. You wont see SD until you get it home, and its probably too late at that point.
A lot of shows are made in SD and broadcasting them on an HD signal doesnt make them better.
A lot of stations broadcast content originally recorded in HD but the channel broadcasts in SD, and that makes it about the same as SD.
A lot of cable channels on high definition digital cable are still analog SD...pretty much everything in the lower 99 channels is still analog and SD.
HD takes up a lot of "bandwidth" in the cable or satellite, so a lot of cable and satellite providers shave down the quality of the signal to make more channels fit. For example on sundays, directv throttles down all the other channels to make more room for the high def football games on the "NFL sunday ticket", and IIRC they also cut back on the more mundane HD channels on friday and saturday nights to boost the quality for PPV movies/events/premium channels. Dish network shoves so many HD local channels into such a small bandwidth that they look worse to me than the analog SD cable equivalents. While offerings are improving, many satellite and cable HD channels are ones most people wouldnt bother to watch very much. Directv and Comcast offer a bunch, but theres only 3-4 I'd watch.
The odds of getting a show filmed in HD, broadcast in HD, rebroadcast by your cable/satellite provider in full resolution and bitrate, and having a set that faithfully reproduces that show...are relatively small unless you primarily watch premium channels or sports.
I forget the exact number, but the amusing stat of the week was finding out that something like 35% of people had hooked their HD set up wrong in a manner that restricted the output to SD and didnt know it.
I've looked at and have owned flat panels, but then discovered that they didnt really save me much because I either didnt have anything critical to put in the floor space beneath them or what I put a cabinet under them to hold the tivo/dvd/vcr/cable/sat boxes that used to sit on top of the tv.
I have all high def sets...a 65" rear projection set, a 55" LCOS flat set and a 32" lcd set and I used to have a 42" plasma. According to my meter, the 65" set takes the least electricity to run and produces the least heat. The picture is viewable from much wider angles. The picture looks as good from 6" away as from 6 and 16'. The LCOS, LCD and Plasma sets all look terrible up close as you can see a lot of digital artifacts on SD broadcasts.
Without my glasses, I frankly have some trouble seeing the difference between SD, 480P, 720P, 1080i and 1080p.
I have a friend with a whole home theater. The seats, tiered floor, black velvet curtains, the big popcorn cart, the fanciest latest generation 1080p projector, and a stereo that has speakers that individually cost as much as many cars I've owned.
We watched a football game there recently. It looked great. Everyone was going up to the extremely expensive screen and noting little odd details about an announcers tie, the pores on a guys forehead, etc. For the first 15 minutes it was very interesting. Then we just watched the game and I forgot about how great the resolution was.
If I were buying a set right now, I'd buy an HD set but I'd read the consumer reports info on which HD sets display SD very well and which dont. I dont think I'd be hung up on a flat hangable screen unless I had a flat place to hang it and that brought me some sort of benefit. It is inadvisable to hang them over a fireplace, since the extra heat, soot and smoke will pretty much cut their lifespan in half. I have no fears about tube and rear projection sets although they're heavier and take up some floor space...they'll probably last longer and they're cheap. Modern rear projection CRT's are a lot thinner and lighter than they were a few years ago. Rear projection LCD/DLP sets are really thin and light.
Some people get headaches from DLP sets. But if you dont and nobody else you know does, they're very attractive from a quality and price perspective.
Dont buy an LCOS set.
Make sure wherever you buy it, that they have at least a 30 day return/exchange policy in case you dont like it.