you CAN set different resolutions on different monitors, you have been able to do so for a long time. i did this on an 8800gts for christ sake. although i wasnt aware the 660 could do 4 monitors. if you can you would probably need active display port adaptor, or adp monitor. i would look into that b4 you buy. and 2500x1600 is pushing it to game on that card, depending on the game and settings you may need to lower the res to 1080p to get good fps.
You can have different resolutions on multi monitors... The only reason you would need the same ones is if you played in Surround or Crossfire.. but from the looks of his post he is only planning to play games off the 2560x1600 monitor. I believe the 660 uses 2 DVI 1 HDMI and 1 DP as connectors btw (and i would assume his 2560x1600 monitor has a DP port already).