Well, it took nearly 2 years for 3DFX to fully lose brand recognition.
But... Contrarly to nVidia, they didn't have parts which were near in performance to their competition. The V5 5500 got killed by the GF2 GTS.
So, first of all, I estimate that in a position where you are still nearly as good, at least to the customer's eyes, it would take more time to lose your brand recognition. At least 2 years and a half I guess. And when is that, exactly?
That's H1 2005. Eh.
And then there's the NV40, and I'd be very surprised if the NV40 wasn't at least nearly as good as the R420 again. My guess right now is, performance-wise, it'd be a mixed bag. And the NV40 would win hands down feature-wise. Of course, that doesn't even mean anything: because a mixed bag is never really balanced...
For nVidia to seriously get in trouble, they'd need to fail to deliver with the NV40. And to nullify the R3xx advantage, nVidia would have to deliver on time, with performance better than the R420, and it should be available at least 3 months before the R420.
And frankly, that last scenario is not very likely at all...