Well it's not up to me to decide whether it's a better buy.
From what I see here
SLI'd GTX260's (and I believe those are 192's rather than 216's) will outperform a 4850X2 by anywhere from 10% to 40% depending on the game.
35% in Far Cry 2
20% in CoD4
7% in Bioshock
31% in Crysis (though back then results were considered slightly dubious in Crysis, I would leave this one out personally)
42% in World in Conflict.
That's all on 1920*1200.
I mean, the competitor of the GTX216 is the 4890, a card that is much more powerful in CF than a 4850X2 is.
I'd take a single GTX216 over a 4850X2 solely because I'm not that big of a CF fan anymore, I never had a CF on a single card though. However, I'm not going to try it out anytime soon either.
Not that it doesn't work, but for instance currently, my CF doesn't work in BC2. Whereas on previous drivers it did work. It stopped working on 10.10 and as far as I can tell, it only loads each card for 50% on 10.12.
It's those things that annoy me.