In general, I prefer a strong single card over dual card setups.
Some games do not play well with sli.
And... there is the issue of microstuttering, particularly with lower end cards.
Read about that here: http://www.tomshardware.com/reviews/radeon-geforce-stutter-crossfire,2995.html
If your budget permits, look at a GTX670 or 7970 which is about as good as it gets for a single monitor.
Past that point, you get diminishing returns for your dollar.
The newer 28nm based cards (GTX6xx and amd 7xxx) need less power and run cooler.
I would look for one of those first.