No new videocard slots, that would be the worst possible move right now. Theres WAY too many people with PCI-E and even AGP right now, introducing a new one would be suicide for them.
As for new generations of videocards, theres the GeForce 8s in late November, by the latest estimates. And ATIs R600 has been delayed into 2007.
If youre buying right now, dont get the 7900GT, go for the 7950GT. Amazing card, with single-slot cooling! More or less the same price too, at $300. And it overclocks to 7900GTX performance so easy, its not even funny.