GTX 780 is def worth the wait. It is x3 faster than GTX680 at the very least from what Nvidia say.
Adding another 6950 is a good option till then.
By then intel will have unleashed their Haswell cpu's (March to June 2013). And that is kick ass tech too.
Damn it your making me going to have to buy another CPU and MOBO..... probably a PSU at that point. I think the 680 then SLI fits my set up the best really. I have 850W PSU which really is enough for that card in SLI(they made it use LESS power). I can wait for my CPU to get dated before upgrading. So HD 6950 late summer. Then sell those and get a GTX 680 Winter 2012-13. And SLI half a year later because of the price drops and better Drivers. First adopters take risks I do not like.