The best way to judge GPU performance is to look at benchmark results for the specific games you play, or want to play. A Google search is all you need to do. The GTX 970 is still very new, so finding benchmark results may be trickier.
A benchmark is simply a measurement to judge performance, like the 0-60mph time for a car.
If you're pricing in USD, you can buy a GTX 970 for around $350 according to PCPartPicker. It's a great card and generally performs better than an R9 290, but the increase is relative. For example, the GTX 970 may give you 90 fps whereas the 290 may "only" give you 70 fps. If you have a 60hz monitor, nothing above 60 fps is registered anyway, so the extra performance of the GTX 970 becomes obselete.