I didn't say it but I thought the quotes you got especially from the independent dealer (if he used the newer Toshiba and sold you the correct Polk models for the center and surround) sounded quite fair. The problem with the BB qoute was the cheap speakers. The online prices at first sound great until you figure, in your case, all the work you have to do.
Masterying01 gives you great info straight from the horse's mouth since he is works in the business. We have others here who either work at dealers or are professional installers.
I would consider the dealer's installation service unless you feel comfortable with the carpentry and hookup work. However, unlike one poster said, there is absolutely no problem hanging any 50'' plasma as long you are able to accuractely locate the studs. You use lags and they will not fall down if installed properly.
There are many ''comparative shopping'' websites that give you prices from many online dealers. One of my favorites is www.pricegrabber.com. To find others and many prices just go to your search engine like Google or AOL, etc. and enter the model number such as ''Toshiba 50HP66'' or for review info enter ''Toshiba 50HP66 review'' (hard to get reviews on very new models).
With sites like pricegrabber just enter the brand and model number or something more general like ''Pioneer receiver'' and then narrow it down further. You will normally find many prices including shipping and any applicable tax after you enter your zip code. Only buy from dealers with at least a 4 or 5 star ratings with at least 200 ratings.
When I got those prices I just did a quick check and grabbed prices from different dealers that I know are reliable. You will find that there are excellent dealers that seem to sell mainly at list price such as Crutchfield, Vann's, etc. Many prefer to deal with these rather than local dealers because they are very knowledgeable, have great return policies and usually charge no tax unless you are in their state.
Other dealers tend to discount more and might possibly not be an authorized online dealer and thus offer their own warranty. Remember that quality speakers and receivers are extremely reliable and almost never have repair problems. Some of the dealers that I have bought from or know to be reliable are, in no special order: NewEgg, B&H Photo, J&R Music World, OneCall, Buy.com, Etronics, Abt, to name just a few. I just went to pricegrabber and entered Panasonic 50'' plasma (because I couldn't remember that exact model numer) and found the 60U model you mentioned, which is a great choice along with the Toshiba. I found 29 prices, including 5 prices in the $3158 to $3185 range from dealers with a 4.5 or 5 star rating with over 1000 ratings. I have saved lots of money over the years buying online, however, you must buy with a credit card only and be prepared for an occasional problem like with any dealer.
Many people are hestitant to buy a plasma online but those who follow the advice above have had excellent results. Many also feel more comfortable buying an extended warranty especially for their plasma. I only buy top brand plasmas and never buy any extended warranties for any product, sort of the self insurance stategy. It pays out bigtime in the end but you have to be a longterm stock market type for this plan.
If you are a complete newbie then be prepared to do a lot of research if you buy online, read all the owner's manual and come back here and ask lots of questions. Also you will have the problems of the DIY installation.
BTW, you will be hit if not already from the dealers with a strong push for extended warranties and expensive speaker wire and audio/video cables and power conditoners/surge protectors. These are big profit items and will be pushed hard. Expensive Monster brand cables are not necessary in my opinion. A quality surge protector is necessary.
Good luck whichever route you take and come back here and ask as many questions as you like.