Without a doubt gonna be 1Gb. Memory size has always double or remained the same from generation to generation. The exception is the geforce 8 series but the g92 showed that nvidia didn't like the weird jump to 384bit and will more than likely go to 512bit. This is the bell ringer for those specs posted at vr-zone being bs.
The real question is shaders. Since unified shaders are brand new we can't really tell what a realistic jump in them would be for a completely new generation. Best bet is to look at the progression of programmable vertex/pixel shaders that started with the g3. From series to series the total amount jumped anywhere from 33 to 100% for g5 through g7 series initially released gtx/ultra card.
So replace 2Gb gddr5 512bit with 1Gb gddr4 512bit and count on 176 to 256 shaders.
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