1) I just bought a Dell u2711, and it is fantastic. Great picture, multiple inputs, good UI, you get what you pay for.
2). If you want 2560 x 1440 or 1600, you're limited to dvi-d or display port at the moment. HDMI 1.4 can support ultra high resolution, but I haven't heard of many success stories yet.
3). Not sure of any 2560 x 1440 or 1600 monitors with 120hz, but there may be some out there. That's a lot of data to transmit. I run 2 480s in sli, which are comparable to a 680, and gaming is amazing. Most games run in high or ultra resolution with no problem, though I had to turn down AA in Skyrim with reality mods enabled. The extra resolution definitely makes a difference in games.
4). Power dvd pro asks if I want to temporarily change resolution to 1080 when I put in a blu ray, automatically reverts on program exit. You can choose not to, the picture will stretch and distort a bit, but plays fine.
5). The higher the resolution, the more you can see on screen. At 2560x1440, I can open up two full size windows of almost any program without anything being cut off. 1440 and 1600 have the same horizontal resolution, you will get more vertical workspace with 1600.
6). No problem with keeping multiple tools for multiple jobs if you have the space and money. FWIW, I've played with 3d gaming, and would take the extra resolution any day of the week, but this is a highly personal opinion.