the 1700x is going to be your best value and its slightly better than the 2600. the 1800x isnt much of an improventment from the 1700x. im running an i7-4790k with a 1080 and my performance is great. the 1700x is just a little better.
why are you even considering upgrading? the 7600k is around the same performance as the 1700x. huge waste of money to upgrade. that chip is fine for gaming with a 1080.