I disagree...at 2560x1440 or 1600 I find a single card to be too slow for a quality experience.
My standards are such that I need two cards even if my first card were an overclocked 7970 I would still not have a good enough gameplay experience.
I recommend keeping the 2nd card.
You'd have to qualify this statement a little. It's not "too slow", it just means that in the most demanding games you'll have to lower 1 or 2 settings. Also, since most games are multiplatform those games will generally always run acceptably at x1600.
If one is comfortable with that they could be happy with one card, a lot of games don't have a huge visual difference between medium and high settings. Personally, i'm in the camp that likes to max everything possible (like you i'm guessing).