Nvidia's strategy is release a Ultra powerfull card, and Wait for ATI's answer.
During the meen time, impulse buyers will sell anything they have to get it.
When ATI does show their hand, nvidia releases a slightly slower one for a cheaper price (100 bucks to be exact). That's what happened with the 6800 series.
This year, ATI has yet to show their hand. Since it will be a while before ATI answers back to nvidia. Nvidia followed through with their strategy. Thus the 7800 GT was born. This puts more pressure on ATI. However ATI is still doing quite well. It's just that Nvidia is now a force to be reckoned with. This just means that new families of GPUs are going to come out every year,
or even more frequently. Meaning the 7800 GT will be next year's 6800 GT.