Some good advice in this thread, as well as some misinformation.
A plasma is going to look better than an LCD in a brightly lit room (generally speaking). LCDs traditionally have a much harder time reproducing deep blacks than plasmas...they're getting better and are almost there, but not quite.
An LCD of the same size is going to cost you more than a plasma (again, generally speaking...and LCDs have come down in price dramatically in the last year or so). It's not a huge amount of price difference, but it might matter to you, depending on your budget.
Don't get caught up in the high end cable voodoo that you'll hear. Hulkrogan is correct, with a digital signal, any $15 cable that performs properly will work every bit as well as a $300 cable. This holds true for audio cables as well. Don't ever spend more than about $25-$35 on ANY cable. Monster Cable is the worst out there right now for ripping off unsuspecting consumers...well, them and Bose...but that's another issue entirely that I won't get into now.
As mentioned by others, don't listen to salesmen, they generally only have their best interests at heart, and couldn't give a rat's behind about you or me. They will try to sell you the highest priced item they think you'd be willing to buy.
As for the difference, picture quality wise between HDMI/DVI and component, it all depends on the signal that you're trying to pass through to your display. If you're putting through a 1080P signal, then you have to have HDMI, component passes through a maximum signal of 1080i. 1080P is the newest defacto standard for HD. Within a couple years, you likely will not even be able to buy a new HDTV that doesn't accept a 1080P signal, so if you want to take full advantage of your TVs capabilities, you'll want to be using HDMI. Also, the two new competing home movie formats, Blu-ray and HD-DVD are both capable of passing a 1080P signal, so again, if you want to take full advantage of the technology out there, use HDMI. If all you're gonna use is up to 1080i resolution, then by all means use component if you want, there's really no noticeable difference.
Number of inputs is really only an issue if you're not using a home theatre receiver. Most receivers sold today have at least two component ins, which while not ideal, will pass an HD signal through to your display. It's also quickly becoming very affordable to get a HT receiver with HDMI switching. If you have a "Home theatre in a box" or don't plan on having any kind of home theatre setup then look for a display with as many HDMI/component inputs as possible.
I can't stand anything audio made by Sony, but for TVs, the simple truth is, they make some of the best there are. It makes sense, as that's where most of their R&D money goes. They do tend to be a little more expensive than some others, but sometimes you get what you pay for, I guess.
I personally own a 42" Panasonic plasma TV and couldn't be happier with it. Panasonic is the way to go if you don't want to pay "Sony" money.
The best advice in this thread was given by Vulcan, visit as many websites dedicated to this kind of stuff as you can and educate yourself. It'll pay off in the end.
Here's another website that can help out the prospective TV buyer:
http://forums.soundandvisionmag.com/
Good luck.
