I'd still get the 780ti. They didn't have the microstutter issue cleared up with the AMD cards at the hardware level until the R9 series, so I wouldn't even consider a multi-GPU setup with AMD prior to the R9 since whatever framerate advantage you might realize with the 7990 will be bogus and negated by the bad performance due to the dropped and runt frames associated with microstutter.
There are many published articles about this phenomenon. Also, Tom's had a 'Ask the manufacturer' session with the GPU people from AMD by which I asked the question about the microstutter in the newer cards being fixed. The answer I received is reflected in the second sentence of my response.