Right now it's easy sailing for Nvidia, and if the r520 has more or less equivalent performance as the 7800gtx, then we'll see some decent prices on both cards. Ati is slaving right now over the 90nm process with the r520, and even if the r520 comes out with only 16 pipes, there are rumors of 24 and 32 pipe cards in existence. Which means about 6 months after the r520 release we can expect the technology to mature enough so that a 24 pipe r580 should be widely available, maybe even a 32 pipe version.
Nvidia, on the other hand has a bigger task ahead. There's no doubt that they will have to move to a 90 nm process to stay competitive in the future. After that's done, they can either ramp up the clockspeed of the 7800gtx and hope it can compete with the r580, or they have to design a 32 pipe card. And I seriously doubt that the 7800gtx has an extra 8 pipes hidden inside. So, Nvidia could potentially be going through the same trouble in 6 months as Ati is having right now.
IMO that's the reason the 7800ultra was cancelled, because the gtx might compete well with the r520, and instead they decided to concentrate all their efforts on the refresh generation to compete with the r580.
Anyways, that's my speculation.