Arky,
A couple of things to note here. Most AV guru's will tell you that 1080P is wasted on TV's less than 50-55 inches. Basically the human eye isn't equipped to differentiate that high a resolution on a screen that small. As for the size, bigger is better, no matter what my wife tells me. At 5 feet I would go for the 40. I sit 1 foot from a 23 inch monitor and wish it was bigger (ahem).
Source switching will depend a lot on the video card you have in the PC and the inputs on your new TV. If your new box has 2 HDMI inputs and your video card has an extra HDMI output, you should be good to go. Most PC's these days have 2 DVI outputs (except for laptops which typically have one), so you may want to make sure your new TV accepts DVI. Once hooked up you should be able to just switch inputs on the remote and play Madden in 40 inches of glory.
Topher
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