First, I would also look at 50" as well as the 42" for that viewing distance. Take a tape measure or walk off the approximate distance when you look at the tvs today and view from that distance if possible. Also try to find a 40-42" next to a 50" and see the difference. It's all a matter of preference, but I would most likely go with the larger screen. You can get great 50" plasmas (or 42, for that matter) right now in your budget from Pannasonic or Pioneer (two of the best IMO) if you shop around, especially off the net.
As far as LCD vs Plasma, etc, you will get fans of both technologies trying to sway you to their side
Personally, I liked the plasma better. I think it has a better picture plus I got the 50" size where plasma still has the price advantage. In the 42" size, there is a lot stiffer competition. The biggest differences to me in the two technologies are: a) LCDs will tend to be brighter, but some say the brightness makes the colors pop a little too much causing them to look unnatural. b) LCDs have a much less reflective screen on them. Plasmas have glass screens like the old CRT units do. If you have lights in the room, they may reflect off the screen, so consider that as to where lighting will be, etc for plasma. c) Plasmas have better black levels as a rule. d) Some LCDs still have motion lag where plasmas excel, so in fast moving sports plasmas will most always have the advantage, although this is improving on LCDs. e) If you do go with the larger screen, look for screen door effect in the LCD models. f) Plasmas are more susceptible to image retention/burn in but again, on newer models this is almost a mute point with even moderate care.
As for 1080p being an advantage in the 40" screen size, I would say no at your seating distance. At 5-6', maybe, but at nearly 10, no. As for your picks in the LCDs, I would go for the Sharp first, Sony second and the Samsung last. My reasoning is because of picture quality (IMO!) and unit quality overall, from what I hear and read.
Good luck with your purchase. In the end, just make a decision and don't look back or second guess yourself. Just enjoy your new TV!