AMD's R9 280 is one of the best-value graphics cards out there, but it's still running on old Tahitii GPU hardware from the last generation. The rumour mill is grinding away at the moment, and we're hearing suggestions that AMD are working on a replacement for that old chip, code-named Tonga.
The new Graphics Core Next silicon is meant to solve the problems the current crop of AMD mid-range cards have, namely that they're both too power-hungry and too hot for mid-range GPUs. The new Tonga GPU is meant to be far more power-efficient, while still being able to deliver good gaming performance.
Some rumours have the Tonga GPU running on a new 20nm production process, but I'm taking that with a big pinch of salt. Nvidia look like they're going to be releasing a high-end Maxwell GPU later this year on their existing 28nm process; I can't see AMD getting the jump and going to 20nm on a mid-range refresh GPU.
Whatever the actual spec of the new GPU's component transistors, it seems likely the Tonga GPUs will slot into AMD's lineup, in a like-for-like fashion, with the current R9 280 and R9 280X. Tonga is expected to arrive with the same 2,048 cores the 280X houses, and the same 2GB GDDR5 frame buffer.
If the new GPUs arrive with the same dollar price, with at least the same gaming performance, but lower temps and power-draw, then we could be looking at some serious mid-range graphics cards. With Maxwell currently doing impressive things in terms of both performance and power efficiency at the low-end, AMD need to get working on their own GPU's efficiency if they want to stay in the game.