Im pretty sure it's safe to buy a GTX970 for your 1366x768 monitor, though it's probably way over what you'll need for games, so you should upgrade your monitor if you're buying this powerful a gpu. Something that would probably still play on Ultra on a 768p screen would be a GTX750ti or a R7 260X, they're way cheaper and could save you alot of money if you're gaming fine on your 768p monitor and doesn't have a need to change it. The GTX970 is made powerful enough for 1440p and 2160p gaming, so it would be a waste of money without a better monitor.
My R9 290 (in the same ball park with performance) can even run 640x480, pretty sure the GTX970 will run 768p.
So i would do one of the following
1: Buy GTX970, buy a better monitor now or in the near future.
2: Buy GTX750ti/R7 260X, keep the same monitor.
Hope this helped you!