I could be snarky at this point and say "Plasma 42-inch" because I personally hate, hate, hate watching TV shows and movies on a computer screen.
All that said, my suggestion is to avoid LED-style screens if possible, especially if you plan to watch anything that is a) not HD, b) made before about 2005, c) animation (unless HD on Blu-ray). I had an LED screen with my previous Mac and I wasn't very pleased with the picture quality at all for watching DVDs or streaming (probably one of the reasons my opinion of doing so is so sour), and animation off DVD was a joke at times (I mean, it looked OK until you noticed the pixelation along sharp edges). I moved to a non-LED screen (I'm not sure what type but it isn't LED) when I upgraded to a new Mac last year and I noticed a marked improvement. I'm sure there are similar options for PC. (I similarly have advised people away from LED for television as well in favor of plasma, for similar reasons: picture quality is lousy for anything not produced in HD or upgraded to HD, and there's also the "Starlost chromakey" effect that it gives by deinterlacing film images and making everything look like old-school Doctor Who shot on video. I saw Alice in Wonderland (Burton's version) on one of these screens and it looked exactly like a cheap ITV shot-on-video production from the 1970s I remember seeing. Which was cool I'll admit, in a nice retro way, but not exactly what I wanted from a 2009 movie; plasma makes it look like a proper film.)
It depends, of course, on what you plan to watch. If your interest is in only watching stuff like Avatar and Tron Legacy and Doctor Who made since 2009, that may make a difference than if your intended viewing list includes Dark Star, the original version of Star Wars: A New Hope*, and Doctor Who made before 2009!
If you're serious about making a PC your home entertainment centre, I can only suggest whichever way you go the bigger the better for the monitor (assuming you're not planning to watch everything sitting at a desk with the monitor 2 feet away from you).
Alex
* Ignore this example if Lucas surprises us and includes the non-Special Edition version on the Blu-ray, which IMO ain't gonna happen.