OCZ ZT is one of the better OCZ psu's:http://www.eggxpert.com/forums/thread/323050.aspx
It will be sufficient for GTX560ti in sli:http://www.realhardtechx.com/index_archivos/Page362.htm
You will do very well in synthetic benchmarks, but sli brings some other possible issues.
1. some games do not play well with sli.
2. You may experience microstuttering.
Read about it here to see if it might matter to you:http://www.tomshardware.com/reviews/radeon-geforce-stutter-crossfire,2995.html
I might be inclined to do things the easy way, and just market your gtx560ti in favor of a much stronger single card .