Well...if you don't plan to use that warranty in the next 4 days, I wouldn't worry too much about upgrading. It's not like you're gutting your cpu. You're just putting in a video card and yes it would make a considerable difference. Wouldn't make your fps go sky high though you would see a definite increase. However, you could run shaders and all sorts of other mods without issues.
Not to mention you could play tons of other games, diversify a little, not that I don't love minecraft.
The only thing is due to this cryptomining craze, the prices of 1070 and other cards that are not 1080 have gone up considerably past what they should be. It makes it not a great time to buy a video card, and if you get 1070, you'll be paying way more than you should, in which case 1080 makes more sense. Up to you but if you're going to buy, go to brick and mortar stores in your neighbourhood for it, don't go online, as the online prices are a lot more vulnerable to these price changes.