Just recently, we posted a story that claimed NVIDIA was preparing to launch high-end Maxwell in the October/November time frame. Apparently, that was generous. The graphics company is said to announce their GeForce GTX 880 in mid-September, with availability coming later in the month. It is expected to be based on the GM204 architecture (which previous rumors claim is 28nm).
It is expected that the GeForce GTX 880 will be available with 4GB of video memory, with an 8GB version possible at some point. As someone who runs multiple (five) monitors, I can tell you that 2GB is not enough for someone of my use case. Windows 7 says the same. It kicks me out of applications to tell me that it does not have enough video memory. This would be enough reason for me to get more GPU memory.
We still do not know how many CUDA cores will be present in the GM204 chip, or if the GeForce GTX 880 will have all of them enabled (but I would be surprised if it didn't). Without any way to derive its theoretical performance, we cannot compare it against the GTX 780 or 780Ti. It could be significantly faster, it could be marginally faster, or it could be somewhere between.
But we will probably find out within two months.