I am planning to run multi-monitor set-up though, I believe the 2 gb on 7850 will be helpful.
If you are only comparing the two then 2 x 7850s in CF on a multi-monitor setup would be much better than a 580. The 580 won't support more than two displays at once anyway. So if you're going multi-monitor at least stick with the 7850. If you are serious about it, you should consider selling the 7850 and going with a 7970 3gb and adding another when you can afford it. The 7970 is better suited overall for multi-monitor setups. But to get back to your comparison - 2 x 7850s is much better than one GTX 580.