During his WWDC 2016 keynote segment, Apple's software boss Craig Federighi briefly flashed a slide listing Mac models that will be able to run the operating system out of the box when it releases publicly this fall (the macOS Preview webpage does not yet mention anything in terms of the minimum system requirements for the new OS).
It seems that Macs manufactured in 2008 and older models won't be eligible for the new OS. But if your Mac is not on Apple's list, an unofficial option might allow you to run macOS Sierra anyway, with some caveats.