Unlike the old Feed, you can fine-tune Discover to show you more or less of a given topic. You won't have to deal with cards showing up every time simply because you found a subject interesting once.

There's no word yet on a wider launch (including international availability). Given Google's strategy, it could be more a question of when you see Discover rather than whether it shows up in the first place. Info cards have been a mainstay of Google's mobile experience for a while, and that isn't likely to change in mobile browsers.