We know to expect the arrival of AMD's first RV770 based graphics card, the 4850 one week from now. Given cards are already beginning to show up in a few stores out there, it's safe to assume they will be available in healthy quantities come launch day.
But as to when we would likely see the 4870 physically show up has been a bit of a guessing game. Word is beginning to spread that this card will also be shipping to distributers next week, though will likely be in limited numbers until early/mid-July.
As for the biggest gun to hit the lineup; the dual-RV770 Radeon HD 4870 X2, it is said that we will likely be waiting for this one to show up in stores sometime in late August/early September.