Is it worth updating my graphics card?

Assuming you're playing at 1080p then a 1070 is good enough. If you plan on upgrading your monitor however to 1440p or 4k then maybe a 1080ti would be a better choice. But even then, going from a 1070 which is already a very powerful card, to a 1080ti is not significant enough of an increase to be worth the extreme cost difference. You're literally better off buying a 2nd 1070 and just running them both in SLI if you ever do decide to upgrade otherwise wait 2 years for a next gen card.