the nvidia card comes with a gaming bundle that is worth at least $100 on ebay, or $150 retail. but the performance is very similar. the 280x might edge it out in some cases but usually not by more than 2 or 3 FPS.
you can't really game across just 2 monitors, games force to use 1 or 3. if you want to use high AA settings at 1080p, than 2GB is a minimum. 3GB would probably be perfect.
Definitely at least 2GB, whether you should get more than that depends on your budget. If you're going to be maxing things out, then some games can exceed 2GB.
the nvidia card comes with a gaming bundle that is worth at least $100 on ebay, or $150 retail. but the performance is very similar. the 280x might edge it out in some cases but usually not by more than 2 or 3 FPS.