will the 28nm cards ALL be more powerful than the previous ranges ?
Nvidia sometimes just rebrand cards.
28nm is a great advantage to graphics card producers. You can get twice as many circuits on the same size wafer with 28nm vs 40nm. That translates into a important cost savings.
Will you see some of that? Probably not. Cards will be sold at what the market will bear at every price point.
At the high end, which we will see first(like the 7970 now, and kepler in april) 28nm will also lower the power requirement which is currently a big problem. You just don't really want to go past needing two 8 pin pci-e connectors. That will raise the top end performance bar, and will enable the vendors to market upgrades to the price insensitive bleeding edge market.
Eventually, though, expect most production of more modest cards to switch over because of cost savings. At that time, expect to get more for your money at every price point.
Expect some confusing number labels. You might expect a 6950 to be stronger than a 5870, but they perform the same. You might expect a GTX570 to be stronger than a GTX285, when they are comparable. For marketing purposes, expect everything to be a 7xxx series, and perhaps higher. For those who know, you can determine the capabilities of the new cards compared to the old via benchmarks and reviews.
And, if you know, you will get fair value at every price point.