From what I've read, prices are expected to drop over the summer as the Chinese are going to be shipping lots of panels, plus the advent of more 3D sets will probably move the 2D set prices down too.
Unless you can get free shipping from an online retailer, or a price that is well below that of the local stores, the shipping will probably eat up the sales tax savings, plus if there is a problem with the set you'll have to ship it back instead of hauling it back to Best Buy or whatever B&M store you would otherwise patronize.
The only time I bought a TV set online, it was on the just-announced-to-be-discontinued Pioneer Kuro Elite plasma. As that was the top-rated TV at the time, prices went up by $600 or more within a couple of days, but I got lucky and found one at a discounted price online. IIRC, the freight shipping was around $150 as it was a pretty heavy TV.
If you wanna shop online, use one of the price comparison tools like Pricegrabber, etc. I'd check Amazon.com prices first to get an idea of the average selling price - usually the Amazon price is about the middle of the list.
"LED" TVs are just LCDs with LED backlighting, at least for the moment. OLED TV's are the next big thing, but then they've been that way for years now
. The advantage of LED backlighting is that they can be dynamically dimmed for an apparent increase in contrast ratio, and also use a bit less power.
As for brands, personally I'd go for Sony, Panasonic, LG, Samsung etc. I'd stay away from Sharp Aquos, as I had one develop a bad panel a few months after the 1-yr warranty expired, only to find out Sharp discontinued that model as well as the replacement parts so basically it was unrepairable.
As for size, that usually depends on how far away you are going to be sitting from the set, as well as your room size. I have my 50" Pioneer mounted to the wall above my computer desk, so I usually sit about 4-5 feet away from it when playing PC games, and 12 feet away for general purpose TV watching since I have a futon sofa. Looks great either way, since sitting so close during gaming it's sorta like having Eyefinity or whatever AMD calls their multi-display setup :P.