one major difference between lcds marked as "monitor" or "television" is in how the pixels are arranged. the arrangement on monitors allows text to be read easier and at smaller fonts. the arrangement on televisions allows shapes to be displayed better.
i believe that all 3d ready displays are 120hz. some tvs are 240hz. if your video card is capable of handling it then yes your 120hz display should work fine.
3d ready and 120hz monitors aren't commonplace yet. your selection might be slimmer. also 24" and below are more commonplace then 26 & 30" monitors. if you want something larger then most likely it will be a tv.
i'm running a 40" sony bravia (60hz version) as my main screen. i can tell you that text is noticibly harder to read than on my old 20" monitor. i have to set the windows scale to larger and the zoom in for IE at +10 to +20% sometimes. in games i always have to set the OSD/Text to large. on a monitor i never had to do this. however, games and movies do play excellent on the display so i'm happy with the larger text size. it all depends on your preferences. if you do work, write, or surf the internet more than play movies/games then a monitor is probably the better choice for productivity.
So even the new 3D TVs aren't accepting more than 60Hz. That sucks. Only the output is 120Hz after image enhancements?
If you recall, I already explained it to you in the video card section about a month ago when someone was considering using a 120Hz HDTV to play games and hoping for up to 120FPS.
120Hz and 240Hz HDTVs does video interpolation to insert additional frames which smooths out video play back. This process takes a little bit of time and causes input lag. Therefore, it is recommended you switch the HDTV to "Game Mode" or 60Hz to eliminate this particular input lag. So 60Hz in 120Hz out.
3D HDTV can be used to watch both 3D movies and play 3D games, but are still limited to 60Hz input. While watching 3D movies the input drops to 48Hz because movies are filmed at 23.976FPS. That means 24Hz per eye. I assume for games it would be 30Hz per eye which means the maximum FPS will be capped at 30FPS.