There will be a 4gb version. It's on EVGA's site. The 680 still beats the 7970 at 2560x1600, but the gap is smaller. http://www.tomshardware.com/reviews/geforce-gtx-680-review-benchmark,3161.html
lets say your buying a 680gtx. When the 780gtx will be released you'll probably want to buy a second 680gtx at a cheaper cost. But beeing locked at 2gb this option might not be available to you depending on the game needs at that time so you might be seeling your 680 to buy a 780.
While someone with a 7970 will just buy a second 7970 and chance are that CF 7970 will beat a 780gtx by a mile.
I'm not saying the 680gtx is a bad card. But some poeple get overexited kinda fast for the little info we do have about that GPU.