Gibbo from OverclockersUK.co.uk claims NVIDIA's GeForce 980 and GeForce GTX 970 reference cards will feature 4GB GDDR5 memory. He notes models with custom coolers and 8GB GDDR5 memory will follow at a later date and claims the 980 will definitely be faster than the GeForce GTX 780.
It may seem a bit strange that these cards won't be faster than the previous-generation but that's due to NVIDIA's messed up naming scheme. The GeForce GTX 780 and 780 Ti are based on the flagship GK110 GPU whereas the GeForce GTX 980 is based on the GM204, which is the successor to the GK104 (GeForce GTX 660/670/680/760/770).
Thankfully, Gibbo from OverclockersUK spilled some beans about GTX 980 and GTX 970. According to him NVIDIA is planning 4GB reference models and also 8GB with custom cooling solutions ‘at later date’. Of course, we also heard about 8GB models. In fact flagship cards with doubled memory are nothing surprising. According to OCUK’s and our information GTX 980 and GTX 970 with 8GB models are expected somewhere between November and December.
Gibbo did not comment on GM204 performance, but he did mention that GTX 980 replaces GTX 780, not GTX 780 Ti.