Ideally, for 47"/46" LCD -> you can differentiate quality difference between 720p and 1080p at the distance 6 ft and below. After 9 ft distance human eye with 20/20 vision can not differentiate quality difference between 720p and 1080p for this size TV.
That means sitting at 12 ft, run either 720p or 1080p both will look almost same for 46" TV. So there is no point in spending money on 46". Either save money and go for 42" or spend more money and go for 60" but this is not advisable.
I'll tell you why.
We have to understand this matter of differentiation properly. For Large TV, if source video quality is not good (cable TV), sitting at relatively close distance will give you pathetic viewing experience. Sitting at relatively far distance will not help you to understand the difference between 480p/720p/1080p and all quality pictures will look more or less same.
Conclusion for 46:
9ft and less: Enjoy only 1080p;
14 ft to 9 ft: Enjoy 720p and 1080p as more or less same;
14 ft or more: Enjoy 480/720/1080 as more or less same
For 42"
8ft and less: Enjoy only 1080p;
12 ft to 8ft: Enjoy 720p and 1080p as more or less same;
12 ft or more: Enjoy 480/720/1080 as more or less same
For complete details please check this link:
Home Theater Calculator: Viewing Distance, Screen Size | CarltonBale.com
Please do a proper demo as these are theoretical figures. I would suggest have 480/720/1080 i and p sample video ready with 24p for 1080 resolution and take demo of 42" and 46" TV at different distances for all kind of videos and decide.
P.S.: I don't have LCD TV and also looking to buy one but with patience. I saw a demo of Samsung LED with 100/200 Hz and was not impressed. A more goes into algorithm which decides where to insert extra frames. This may not be accurate for non-linear movements of the object as well as objects changing shape on TV.