The new single-GPU flagship GTX 980 is no slouch.
VC has test numbers for a number of different Nvidia 900 series parts, including three different GTX 980 cards with varying clock speeds (the site does not know which is the reference design, hence posting all three). Depending on the clock speed, the GTX 980 could sit right above, or right below the current single-GPU champ GTX 780 Ti. Regardless of the clock speed, the GTX 980 will likely come with 4 GB of GDDR5 memory on board.
Also included on the desktop side is the GTX 970, which should also come with 4 GB of GDDR5. This card, depending on the game, will sit somewhere around the GTX 780 and original GTX Titan.
Several mobile components are also shown in benchmarks, including a GTX 980M with 8 GB of video memory on-board. The GTX 970M part also gets a RAM boost, to 6 GB.
All the posted benchmarks are from Futuremark -- no games like Bioshock, Crysis, etc. -- so real-world performance has yet to be seen.