I don't think they're going to launch any card the Sept 10th, it's only the date of a press event under NDA at US (and I think that today is the date for the same event in Europe). More than probably, both HD5850/HD5870 are going to be launched at the same time, as usual, and there's rumors about more cards being launched simultaneously to cover other market segments, but I don't know. The fact is that very little is known about that cards, the secrecy around this launch is surprising to say the least...

Regarding the question about this gen cards being enough "until something big changes", I think that videoconsoles are what limit that aspect more: there are practically no PC exclusive games aside online multiplayer (because of their little profitability), and online multiplayer games are usually targeted to very afordable hardware specs (the more players, the better the value of a multiplayer game, so publishers have a higher than usual need to target the wider possible audience). Chances are that there's not going to be any substantial changes until the next videoconsoles generation is released.