"because the 7850 will last till then"
If your present graphics performance is satisfactory, then you must wait. Two things happen over time:
ONE: Computer hardware becomes cheaper
TWO: Computer hardware becomes faster
Six months from now 680's and 7970's will be cheaper as will 690's and 7990's
The new generation of 790's and 8970's will be out or coming.
Your best bet is to wait until your present graphics performance is not acceptable and upgrade. Perhaps go from single card to SLI/Crossfire or to new much more powerful card.
With a 1080 display, even a 670 or 7950 should be plenty as would crossfiring your 7850's. 690 or 7990 would be overkill and a waste of money.