If you look in the left column they list all the deals that are applicable to that product. When I bought my 42" in the summer I got a free 5.1 + dvd system that is still in the box, waiting for an opportunity to be re-gifted I guess. You'll likely get the same $100 off no matter what TV you bring in - get an old busted one from a curb somewhere and use that.
The Westinghouse LCD TV's are cheaper because they don't have an "HDTV Tuner" built-in - but in North America we don't use the HDTV broadcast standard, so each HD service comes with it's own HDTV tuner (like your rogers box, or your DirectTV Box or whatever). So the westinghouse is essentially a big, fancy monitor... which IMO is the way it should be.
The quality of the LCD panel is as good or better than current LCD monitors - one of the reasons why they're not as expensive as an equivalently sized LCD monitor is because the pixel density is much lower (and therefore easier to fabricate). In fact, a 24" 1920x1080 LCD is about $800 on average now... sizing up to 37" is only $300 more for the same resolution! And you get WAYYY more inputs, plus a remote!
I did a lot of research into HD and TV's before I bought this one, and I still have yet to find a better deal anywhere (and I still look!).
Best Buy and Future Shop, although owned by the same company, are managed separately. This is nice, since if one brokers a great deal one something, has a massive sale and clears out, you can get the other to price match

I've also gotten Future Shop to take a Best Buy gift card before!
Best purchase I've made in the last several years - I can barely look at old TV's now! And it uses less power than my old one too
As for size: sit where you watch TV and make a fist and the end of your arm like you're holding a milk jug at arm's length. The perfect size for a TV is about as tall as your fist (give or take) if you get too much bigger you won't be able to focus on the whole screen. If you're within 15' of your TV, 55" is too big for comfort. At 8'-10' a 37"-42" is about right.