This is rather an important point. How close to this supposed 42" full HD screen is the viewer proposing to sit?Full HD on a 42 inch is only nice if you're going to sit within 1.5x or below the screen size.
The optimum viewing distance for such a display is to have your eyes just five and a half feet from the screen. Are you really planning on sitting that close?
At a distance of a bit less than 8 feet from a 42" screen, a person with normal eyesight is incapable of seeing any difference between a 1920x1080 display and a 1366x768 display. So, if you're planning to sit 8 feet or more from a 42" screen, then paying for a full HD device is a waste of money.
At a distance of a little more than 12 feet from a 42" screen, a person with normal eyesight will see no difference between a 1920x1080 display and an 852x480 standard-definition plasma.
Personally I think HD is pointless unless you have a decent sized screen. The woeful lack of decent HD televisions in Britain, combined with budgetary restrictions, mean that I've ended up with a 55" SXRD rear projection model. This is a good deal: it performs very nearly a par with full HD resolution plasmas from Panasonic and Pioneer, while costing barely a quarter of the price (at UK prices). 55" still isn't really big enough for HD; if I could have afforded the 70" version I would have got it. (Optimum viewing distance for a 70" 1080p screen is about 9 feet). But I certainly wouldn't want to go any smaller than what I have.
If you do decide to go bigger, then you should definitely look into rear-projection (which is far more widely available in the US than it is over here). Not just SXRD but also DILA (which is JVC's name for what Sony calls SXRD) and DLP.