I have a very similar system, two 3gb gtx 580s, i5 3570k @ 4.5 and run a 2560x1440 monitor. To answer your question, you will get 60fps in very light missions, and flying over areas not well crowded. You'll still dip a lot, sometimes in the 30s.
It's more than playable, this is with 3gb 580s though. Just keep that in mind, the higher the resolution, the more vram you will use (typically.)
I am sure you'll still be happy with your performance, the increase in screen real estate is great and typically monitors that have 2560x1400 resolution are very good IPS panels.