It will be enough for the first few dungeons.

To be clear, the 660 now will be the next poetics so by farming now you will gain on poetics when the expansion will be released. But poetics will only have the upgraded one, you will to buy an entirely new items or do the 3 raid 24 to upgrade an old non upgraded item.

And to be complete on the gear during the expansions.
First quests will give you the 645 ilvl gear, then the first dungeon will give you 650ilvl gear, next quests will have 655 and next dungeon 660.
So with 640 ilvl gear, you will take all items you will found as an upgrade and with 660, you will only have upgrade starting at the 95 dungeon and 660 will be largely enough to go to 100 but not the 640.