How soon is "in the future"? I find 2nd GPU upgrades only viable if it's within a year, because by then, the next generation is usually on the horizon with something even better for what you could've paid for. Usually.
Case wise, it depends on how much space you're willing to give it. If you're going with MidATX cases, you should find something with breathing room in the middle for the graphics cards. While I won't stop you, I don't recommend a FullATX case mostly because it's a waste of space. Unless you plan on sticking 10+ drives in there.