I would venture to say that it IS impossible to tell. You are sitting very close, but the pixels are so close together on those 22-24" LCDs that it will be impossible to see the difference.
If you're going to watch a movie on it, then you'll probably sit back a little, and the further you are away the less likely you are to notice, so don't worry about it.
I wouldn't use HDTV resolutions as a deciding factor in my own PC monitor purchases, but I don't use mine to watch tv or video on very much. Maybe an HD Lost here and there, but that's about it.
One thing you should consider is that the 6800 is going to choke on 1920x1200. I would select a monitor somewhat more suited to your video card capability. Otherwise you'll be constantly downscaling games from the LCD's native resolution, and if you're doing that, then how can you justify the extra cost for more pixels that you aren't using?