120fps at 1920x1080 i can believe. if you stay at similar resolutions then performance should be fine. its only until you start doubling the resolution that your fps drops by 50-75% or more.
a typical signal is 60hz, 120 is achieved via dualink only. you really dont need it unless you want 3d (on a 3d capable monitor). the signal doesnt vary, its always 60hz no matter what resolution (well in europe i think they use 50hz but thats besides the point) the point is that whatever the rating is, it is a constant. your frames per second (fps) can vary, but the actual signal to your monitor does not.
led/ccfl are equal. there really isnt any benefit to going led except that leds *may possibly* last longer and that they are a little more energy efficient but thats about it. the fact that they tinge the screen blue (which you can adjust out) makes me recommend ccfl still. but either will work.
at 27" the difference between 2560x1400 (dell) and 1920x1080 (sams) is absolutely huge. the dell will be much more crisp and clear (taking into account what i said in my last post) than the samsung. that said, i personally use a 40" 1920x1080 television as my screen and i enjoy it. the ppi is a little low but since i use it as a media/gaming pc everything is kosher. i'd love to have a u2711 but i don't want to throw money out for both (and i really wanted a big screen tv).
the samsung for $100 cheaper is absolutely no competition for the dell u2711 and what it brings to the table (unless you were looking for a 1080p screen instead, in which case you might consider it). personally i'd lean towards the dell.
if you have bad eyes you might have to raise the zoom level up on the dell monitor as text might be a little small (but that might be countered by it being a 27", i'm not sure) but this is not a real issue. you would be able to browse multiple pages at once on a screen (and even game/surf on one screen if you wanted). the downfall is that if you wanted to fullscreen a game at 2560x1440 it takes a beefier system to handle it (though if you get 120fps already you *should* be fine. do look at the video card charts @ that resolution though to be sure fps wont drop too much. DO NOT SKIP THIS STEP.
----
in summary, keeping all of the pros/cons i stated earlier into account, the dell u2711 should be a good choice. at $100 above the other choice you mentioned its a no-brainer.