Gee, you guys must have huge houses if the TV is 4+m from the Lounge.
As far as using a 720p in that size .... because you can't afford the difference for a 1080p TV, or are unwilling to drop the size to a 46" or for whatever reason ...
Generally a 52" 720p LCD will be pixellated using 720p, the 50" plasma would be the better choice @ 720p ...
In regards to projectors ... a few months ago, I was dragged by my wife (and her friend) to watch Prince of Persia at a Cinema with lots of smaller projection rooms (less than 60 people in any room), which was using a 720p projector on a 4m wide screen ... we were sitting about 6-8 metres from the screen and I was watching the pixels squiggle across the screen for the whole movie ....
My wife and her friend didn't notice it, but it drove me nuts .. well more crazy than usual anyway.
I'd suspect that newer projectors do a much better job of smoothing those pixels .. but .. urgh .. I found it extremely d
If the OP is playing low quality Divx's, this will be a major difference between different TV manufacturers & models.
If he's just watching bluray, he might not find much difference - although I can discern a massive difference, and other peoples TV's drive me up the wall because they're obviously blind.
At the end of the day, the OP needs to figure out what he uses his TV for most or intends to use it for most, and then take that with him and test each TV he is considering.