amd radeon ryzen dual-gpu

I'm seriously considering a mid-high end new system, with an AMD Ryzen 5 1600(x?) (or maybe 1700, not x) and _either_ a Vega graphics card (once they're released, later this quarter, but with what specs?!) or perhaps a _pair_ of the new Radeon 580s. I realize that I'm a bit ahead of the game...