i've seen several articles that seem to confirm that ATI is having serious problems with R520. Apparently the product taped out in OCTOBER, yet they are still far from release. The word is that they physically have 32 pipes, but they can only seem to reliably run 16 at the moment.
Additionally, if it does indeed contain unified shaders, as does the R500 XBox 360 chip (a 24 pipe chip, btw), then that changes the comparison dramatically. A 16 pipe solution, for example, might actaully be slower than current X800 series chips, since it has 16 pipes INCLUDING its vertex shaders. So, even if ATI's unified pipelines are more efficient than non-unified designs, you still have to take into account the TOTAL number of pipes when comparing. A 32-pipe R520 has the same number of total pipes as a 7800GTX (24 pixel + 8 vertex). The only difference is whether or not the unified architecture is more efficient - it might be, but i don't think it will make a very big difference, at least until the ratio of vertex shader ops gets to be close to that of pixel shader ops.
I think it will shake out like so - nvidia will be launching the 7800 series this month. Assuming delays getting them into the channel (although nvidia has gotten remarkably better at this with launches like the 6600GT, compared to ATI's X850 and X700XT problems), people will get their hands on boards sometime in july, maybe august. ATI may release a 24-pipe part within the august-september timeframe, but it may or may not be feasible, since it only makes sense if the pipelines are not unified (i.e. 24+8, not 24 total), or if the unified architecture brings big boosts, or if clockspeeds are up significantly. Either way, the full fledged 32-pipe bad boy will probably not show its head till october. When it does launch, nvidia already has a spot ready for its 7800 Ultra, which I'm sure will be clocked just high enough to compete with ATI's top R520. So it's looking like nvidia is going to be in much better shape to regain marketshare this time around, especially if they come out with solid $200-300 offerings like last time around.