Ya I was thinking that to. But I'm curious, what headaches?
If a single card will do the job, I think that is the way to go.
SLI synthetic benchmarks will look wonderful, but
read this on microstuttering:http://www.tomshardware.com/reviews/radeon-geforce-stutter-crossfire,2995.html
If you went to sli, would you need to change to a sli capable motherboard?
Is your case prepared for increased cooling load?
Is your psu at least 700w?
If you need more graphics power, make the upgrade a significant one.
Otherwise, you may be disappointed if you don't see a big improvement.
Your GTX560ti is a very good card. I think you should be looking at something like a GTX670.
Possibly the upcoming GTX660 will be very strong, and may well do the job.
We will see soon.