GPUs with 3 and 4GB of RAM are fairly new to the market, and they will start to see that be used very soon. It will not be long before 6, 8, 10, and 12GB cards are on the market.
Have you looked at a GeForce Titan board? This is the end of the line as far as memory is concerned. It will be a long time until we see cards with more than 6G memory. Probably die-shrinks will allow 4GBit chips, maybe some form of stacking (with ingenious cooling) will allow more. Not going to happen this year.