You could try to get another 560ti, as the 650 apparently can't be set up in SLI. Or go with a single gtx660ti, that way you avoid potential problems such as microstutter, driver issues, or scaling issues that can be present in certain games when running in a multiple GPU setup. Also I doubt that they make waterblocks for the 650, and for the price of the waterblocks themselves I would simply get a better, non reference card (custom cooler). http://www.anandtech.com/bench/Product/547?vs=647
The gtx660ti does perform close to twice as well in certain games than a stock 560ti, so depending on how well a second gtx560ti would scale, it would be a good choice to go with the gtx660ti.
Hope this helps!