Two 560 Ti in SLI is a very powerful setup, especially at your resolution (1600x900), which means that the relatively low VRAM won't hamper framerates. You also shouldn't have as big a problem with microstutter at that lower resolution (it's a worse problem when each card is struggling).
I would probably still wait a few weeks to see how the next generation of nVidia releases affects prices of the 5xx series. It's not like it's urgent for you to get the second card. You will probably see price drops akin to what happened to AMD's 6870, so wait a bit, then grab another card.
OTOH, if you are planning to go higher resolution some time soon, then you might consider getting a totally different card.