Farapon :
One 980 now and another on the far future
The only problem with that is by time he wants to get another 980 some one will say 'no, get the 1180 now and another one in the future' it's a never ending cycle of don't go with SLI get the latest one now and go SLI in the future. Except that he already has a 780ti and this is his moment to get another one.
So I say grab another 780ti and enjoy the power. Once the 780ti SLI is no longer relevant grab the most powerful single card solution thats out.